* Functionality
  * Add probes to LIF cells.
* Docs
  * Remove errorneous statement(s) about LIF cells (there never was an E_reset...)
  * Move probing chapter one level up (concepts/cable_cells -> concepts)
* Tests
  * Add tests for LIF probes

# Introduction

Plasticity processes are mediated by signalling ions, eg Ca++, which are generated by 
synapses upon reception of a spike. This adds a quantity `Xd` for any ion `X`
- initialised as `Xi`
- read from and written to by NMODL density and point mechanisms.
- propagates according to a diffusion law `∂_t Xd = ∂_z c X ∂_z Xd + iX/qi`.
- in contrast to `Xi` and `Xo` there's no buffering and the update in mechanisms
  occurs atomically (and at a different time)

More details can be found in the documentation.

For the future there are some low hanging optimisations
- per-ion conductivity for the matrix solver could be disabled if no diffusion is computed
- cable and diffusion solvers store duplicates of the solver state, could be merged

Closes #1651 

1. All s-exp parsing has migrated to `arborio`
2. CV policies can now be read from string as per #1334. This is needed for the GUI project.
2. No longer can `arb::locset` and `arb::region` be constructed from mere strings
2. We have `arborio::literals` to construct `region`s, `locset`s, and `cv_policy`s in the form of `_rg`, `_ls`, and `_cvp`.
3. The `_lab` suffix now constructs an object that converts to `named` morphologies, rather than a simple string.
4. Constructors of `region` and `locset` are now properly guarded
5. Consequently, a lot of missing inheritance from `region_tag` and `locset_tag` was retrofitted.

Closes #1334 
Closes #1419 

* Change implementation of current clamp from a builtin mechanism to
  functionality provided by the back-end shared state classes.
* Extend clamp functionality to cover more use cases: it now supports DC
  and AC stimuli governed by a piecewise-linear envelope.
* Update Python iclamp wrapper to suit; add unit test.
* Account for total applied stimulus currents separately, and remove
  their contribution to the reported transmembrane current and
  ionic (i.e. non-capacitive) transmembrane current.
* Add new probe specifically for tracking total current stimulus in a cell.
* Update docs, and add an entry for current clamps to the concepts doc.
* Remove entirely the builtin catalogue.

Major changes in Python library and documentation:

* Add global state (immutable after initialization) for the Python module that
  manages the mapping between probe metadata types and the method by which the
  corresponding [sample data can be collected and presented as Python objects.
  This is necessary for decoupling the implementation of the simulation wrapper
  from that of the various cable cell probe types.

* Wrap each of the C++ cable cell probe types with a Python function that
  returns a `probe_info` object.

* Add code for converting and accumulating C++ probe metadata and sample
  data, registered in the module global state against the metadata type info.

* Make the `arbor.simulator` object responsible for recording all spike and
  sample data, with corresponding new methods.

* Use NumPy arrays and structured datatypes for returning spike and
  sample data.

* Place Python schedule proxies under an abstract interface so that consumers
  of the proxies can be made generic over schedule types.

* Add unit tests for the `arbor.simulator` class and its new functionality,
  including distributed spike collection.

* Rework all Python API documentation concerning probes, sampling, and spike
  recording; add information on newly exposed cable cell probe addresses.

* Add new python example `single_cell_recipe.py` which is a generic recipe
  version of `single_cell_model.py`.

* Adjust other code in the wrapper and examples to accommodate these changes.

Minor changes in Python library:

* Add equality tests for `arbor.location` and `arbor.cable` Python classes.

* Use the `py::` namespace shorthand more often in the Python wrapping sources.

* Add an implicit conversion for a pair of numbers to `arbor.cell_member`, so
  that e.g. probe ids can be specified simply as `(gid, index)`.

* Add an implicit conversion from a tuple to `arbor.mpoint` so that a
  morphological point can be specified simply as `(x, y, z, radius)`.

* Modify the interface to `arbor.regular_schedule` so that the overloads
  correspond more closely to the C++ API. In particular,
  `arbor.regular_schedule(dt)` now describes a schedule with times 0, dt, 2*dt,
  etc.

Minor changes in C++ library:

* Change test in FVM lowered cell implementation and exception message for
  `bad_source_description`: assert number of detectors is exactly the number
  of sources promised by the recipe.

* Add comment describing `probe_metadata` object in `sampling.hpp` which
  asserts that the type and value of probe-specific metadata completely
  determines the correct interpretation of sampled data provided to a callback.