Convert currents to current densities in FVM (#381)

Update the FVM formulation to use current densities instead of currents.

Modifications to modcc:
* Update printers to store and use weights for point process mechanisms,
* Scale ion species current contributions by area proportion, similarly to contributions to the accumulated current.

Changes to FVM code:
* Update weights calculation for density and point processes mechanisms:
    * density channels use relative proportion of CV area, i.e. "density",
    * point processes use the reciprocal of the CV area to convert to a density.
* Add `cv_area` parameter for matrix constructor, which is used by matrix assembly to convert current densities to currents.
* Update stimulus implementations (gpu and cpu backends) to contribute current densities.

Other changes:
* Update unit tests to use new interfaces.
* Update units section in LaTeX docs.

Fixes #374.