Expose diverse probes in Python API. (#1225)

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 tha...