Expand solver parameter 'materials' acceptable inputs

[bch0w]

Currently the materials parameter belonging to the Solver module accepts keyword arguments to determine what model parameters to update. That is, materials=='acoustic' leads to only updating Vp, where as materials=='elastic' will update Vp and Vs.

This is correct but may also be confusing: if a User knows they want to update Vp only, they have to look up that the relevant keyword is 'acoustic'.

SeisFlows should allow users to input a comma-separated list of variables to update, which need to match against the SPECFEM output variables. For example materials=='vp,vs,qmu'. I think the old method should be left in as well, for backwards compatibility, but also for Users who approach parameter update from the perspective of acoustic vs elastic materials.