Add "const" to some methods.

This affects IceBasalResistancePlasticLaw and EnthalpyConverter.
Address the code part of #266.

Make "beta", "taub", "taud", "taud_mag" diagnostics available regardless
of the chosen stress balance. Implement "taub_mag".
Add "taud_mag" to the User's Manual.
Split tables in the manual. Rename some variables. See #266.

cbar  -> velbar_mag
cbase -> velbase_mag
csurf -> velsurf_mag
cflx  -> flux_mag