Half cell model voltage hold potential incorrect

[lightningclaw001]

A slight technicality for people who want to do voltage hold on half cells. For half cell models the voltage pulse should only be on the solid electrical potential. However, the half cell concentrated solution models include the kBTln(a_l) calculation implicitly in the electrical potential. This is technically incorrect but only a problem for people who are using the half cell voltage holds with the concentrated electrolyte model (probably only me right now), but keeping track of this in case people do see this problem in the future.

E.g. in the function calc_mu_O in mod_electrodes.py,
mu_lyte = T*np.log(act_lyte) + phi_lyte
should be true for both the SM and dilute models.

[lightningclaw001]

This is a very quick fix but may break some half cell tests slightly.