change units of compute_current

[huangziwei]

To align with jaxleyverse/jaxley#461, compute_current() in all channels now returns current in units of mA/cm^2.

closed #47

[michaeldeistler]

Thanks Ziwei! I don't think we should have a surface_area parameter though. You can access length and radius from within every channel (via `params["length"]), so you can just compute the surface area directly.

[huangziwei]

hmm.. should it be scaled with the compartment size of our model? I think the conversion factor is the surface area of the cell (or the outer segment of the photoreceptors) that those paper recorded from (and it will vary depends on the species, etc.)...

[michaeldeistler]

If the cell is a point neuron then compartment size and cell size are the same.

If a cell consists of multiple compartments then I would argue that one wants compartment size. (and if the channel exists in all compartments then it will anyways sum up to the cell size)

[huangziwei]

makes sense. now the scale factor is computed with the compartment length and radius.

[michaeldeistler]

Thanks!