Option for voltage/current variable names and frequency response of internal variables

[whileman133]

@rtimms @noelhallemans, in my research group at the University of Colorado Colorado Springs, we've defined lumped-parameter LIB/LMB models with slightly different variable names ("vcell [V]" instead of "Voltage [V]" and "iapp [A]" instead of "Current [A]"). Rather than reference "Voltage [V]" and "Current [A]" directly in the EISSimulation class, could these names be passed to the initializer for improved generality with model definitions?

Also: we'd find it useful to evaluate the frequency response of a cell's internal variables (in addition to the cell-level impedance) for comparison with our transfer function models. Do you have thoughts on what changes are needed to support this?

Thanks!