Rename some variables in the code

[Raymond Smith]

Change kappa in the electrolyte to sigma.

Change mcond in the bulk solid to sigma_s.

[Edwin Khoo]

Should also make sure the names of variables and functions align with PEP 8 as much as possible.

[Raymond Smith]

Note that PEP 8 doesn't seem to make much of a recommendation for local variable names, which is what this issue is actually about. We could make another issue for renaming methods and class definitions to better align with PEP 8.

[Raymond Smith]

Fixed in commits 96172ce, 248795e, 889a5a1, 62726df.