Renamed InvSSATikhonov to more abstract TaoTikhonovProblem, added some documentation, and removed explicit relationship with InvSSAForwardProblem.  TaoTikhonovProblem can then be used for alternative inverse problems (e.g. hardness-> SSA velocity).  A Tauc->SSA inversion problem is now defined in SSATaucTikhonovProblem, a subclass of TaoTikhonovProblem.
Merge branch 'dev' of https://github.com/pism/pism into dev
Documentation of TaoTikhonovProblem.