Extended USEION VALENCE support.

* Augment modcc parser to accept identifier or signed integer after VALENCE.
* Treat identifer after VALENCE as global scalar, tied to ionic charge.
* Add ionic charge field to mechanism ion parameter pointers, replacing scalar in shared state with a length-1 array.
* Verify presence of ion name and check ion charge against numeric valence in mechanism info when discretizing in `fvm_build_mechanism_data`.
* Pass ion info, coalesce_synapse global state to `fvm_build_mechanism_data` just via cable cell global properties.
* Add more unit-test only mechanisms (test_ca_read_valence, test_cl_valence).
* Add unit tests for valence verification and reading into mechanism state (multicore only).
* Update existing unit tests for new interfaces.

Implements #780.