Merge analyticalproblems and discretizers

[sdrave]

This pull request does the following:

• InstationaryAdvectionProblem(...) -> InstantionaryProblem(EllipticProblem(...), ...)
• discretize_parabolic_fv + discretize_nonlinear_instationary_advection_fv -> discretize_instationary_fv
• rename:
  • discretize_elliptic_fv -> discretize_stationary_fv
  • discretize_elliptic_cg -> discretize_stationary_cg
  • discretize_parabolic_cb -> discretize_instationary_cg

I would still like to give EllipticProblem a better name. Any ideas?

[sdrave]

There seems to be no really good name. We decided to go with StationaryProblem as 'not specific enough' is better than 'too specific'.