Check that mechanisms have the right kind

[noraabiakar]

• Expose arb_mechanism_kind to mechanism_info.
• While building mechanism data in fvm_build_mechanism_data, check that synapses, painted region dynamics and reversal potential methods have the expected arb_mechanism_kind.
• Add unit test.

[thorstenhater]

Looks good, but please add a kind property to the Python interface
of mechinfo and extend the docs accordingly.

[thorstenhater]

👍