generalised stimulus (#126)

feature: #67 (GPU Support)

Implement stimulii as part of the mechanism framework, as described here: #87 (comment)

* A hand-written stimulus point process derived from `mechanisms::mechanism` was written for each back end. 
* The lowered `fvm_multicell` type explicitly constructs a stimulus mechanism if there are any stimuli attached to the its cells.
* This mechanism is added to the other mechanisms in the lowered cell, so that the update of current is performed in the current update loop (i.e. via the `nrn_current()`) method.

This isn't an ideal solution: we still have a hard coded stimulus type in the lowered cell, however the stimulus is now "in the right spot", and we can refine this better when we work on a better design for generalised mechanisms (i.e. when we have figured out what we going to do).

fixes #104.