Voltage Processes (#2033)

Add the VOLTAGE_PROCESS mechanism kind to modcc, allowing for direct writing to the membrane voltage
Obviously these are extremely sharp tools and can break the cable model, so we add some constraints to their
use (see docs). Still, user discretion is required.

As a sneak peek for what this might be useful for

* implementing voltage clamps
* limiting membrane voltage (irritatingly also a kind of clamping)
* faking ABSTRACT_CELL like behaviour (although that might require a bit more work)

Closes #1343