explicit icephys gain docs (stimulus)

[lvsltz]

To complete NeurodataWithoutBorders/pynwb#1128, explicit gain docs should be added to CurrentClampStimulusSeries and VoltageClampStimulusSeriesas well.

So the units should be: CurrentClampStimulusSeries : Volt/Ampere VoltageClampStimulusSeries : Volt/Volt

I would also update the PatchClampSeries gain units from Volt/Amp (v-clamp) or Volt/Volt (c-clamp) to only Volt/Ampere or Volt/Volt because you have both ratios in both clamp modes so the present brackets are inaccurate. I'd suggest replacing Amp with Ampere in all 5 PatchClamp classes.

cc-ing @t-b to confirm

[bendichter]

Thanks for the suggestons, @lvsltz! I'm moving this over to nwb-schema since it is suggesting schema changes