Extend repertoire of cable cell probes. (#1011)

* Extend repertoire of cable cell probes.
* Add the following scalar (i.e. corresponding to just one raw value)
probe address types for cable cells:
   - `cell_probe_membrane_voltage`, replacing `cell_probe_address::membrane_voltage`;
   - `cell_probe_total_ionic_current_density`, replaing `cell_probe_address::membrane_current`;
   - `cell_probe_density_state`;
   - `cell_probe_point_state`;
   - `cell_probe_ion_current_density`;
   - `cell_prone_ion_int_concentration`;
   - `cell_prone_ion_ext_concentration`.
* Extend backend class interface with `mechanism_field_data` method, that returns a raw (device) pointer to the per-instance values of a mechanism state variable.
* Make unit test for probes backend-generic.
* Add unit tests for new probes.
* Modify `fixed_ica_current.mod` unit test NMODL module to be more generic in naming, so it can be better used in probe unit tests.
* Update examples for new cable cell probes.
* Update Python interfa...