Closes #1547.

- Move `MechCat*Iter` to module instead of catalogue class
- Add `keys`, `values`, and `items`
- Fix a typo.

* Raise KeyError in catalogue.
* Raise KeyError in label_dict.

First phase of dynamically loaded mechanism catalogues.

- Add functionality to load and use catalogues from DSOs in Python and C++.
- Add documentation and support scripts to build catalogues DSOs.
- Re-factor CMakeLists.txt for mechanisms
- Limitations
  - Needs (compatible, preferably identical) arbor sources when building catalogues
  - Can only work on MacOS and Linux (uses dlfnc)

Introduce two minor changes to the Python API
to handle mechanism_catalogues idiomatically.

Instead of

import arbor as A

cat = A.default_catalogue()

if cat.has('hh'):
  print("Found HH.")

for mech in cat.keys():
   print("*", mech)

we can now write

import arbor as A

cat = A.default_catalogue()

if 'hh' in cat:
  print("Found HH.")

for mech in cat:
   print("*", mech)

which is closer to the expectations of Python users.

Extend Ptyhon recipe wrapper to support setting of global cell properties.

* Rename `pyrecipe::get_probes` to `py_recipe::probes`
* Remove `global_properties_shim` : It's easy to make mistakes with the class, as it holds a `mechanism_catalogue` and a `cable_cell_global_properties` which holds a pointer to the catalogue. This would likely have caused issues with the users.
* Expose `py_recipe::global_properties`

* Add interface for querying names.

* Add derived mechanisms to list, clean-up.

* Add unit tests.

* Add python bindings and docs.

Add a catalogue of mechanisms used by the BBP models.

* Substitute `std::optional<T>` for `arb::util::optional<T>` for non-reference types T.
* Remove reference-deducing `util::value_by_key`; replace usages with new function `util::ptr_by_key`.
* Add some missing header includes that were required but included only transitively.
* Remove `operator<<` overload for optional in python/strprintf.hpp; replace with utility wrapper class that catches `std::optional<T>`.
* Wrap some `std::optional` values with `to_string` in ostream output in python lib.

Fixes #1154.

Rename import method to avoid Python keyword collision.

* Add import method for mechanism catalogues, allowing mechanisms from a second catalogue to be incorporated with a prefix, together with Python interface.
* Expose `global_allen_catalogue`.
* Extend unit tests to cover mechanism catalogue import and check for name collision error cases.

Update core library:
* support derivation of named mechanisms in catalogue from a string description of the parent.
* fix bug in radois_ge region expression that was finding region_lt.
* fix test for radius_ge region expression

Add support for mechanism, global properties and catalogue shenanigans to python wrapper