Repace 'invert_options' by 'solver_options' (second try)

[sdrave]

This pull request contains the following changes:

• options is removed from OperatorInterface.apply_inverse
• invert_options is removed from OperatorInterface
• solver_options is added to OperatorInterface
• solver_options can be given to the implict Euler algorithm.

If not None, solver_options is a dict which can contain the keys
'inverse' and 'jacobian'. The following rules apply:

• OperatorInterface.apply_inverse uses the 'inverse' entry
  of solver_options for solver selection and configuration.
  If not available, or None, defaults are used.
• OperatorInterface.jacobian uses the values of the 'jacobian'
  entry of solver_options as solver_options for the Jacobian
  operator. If missing, solver_options is set to None.
• OperatorInterface.assemble returns an assembled operator with
  the same solver_options.
• OperatorInterface.projected and OperatorInterface.restricted
  return operators with solver_options set to None.

This PR addresses #122.

[sdrave]

After some discussion, we decided that there are too many corner cases where the approach in #183 does not work out very well.

As there are relatively few places, where new operators are created, for which appropriate solver_options cannot be known at this point, it seems better to make the solver_options used for these new operators configurable (see 1e54111).

[ftalbrecht]

Very nice, thank you! If I remember correctly, we also decided to allow for an 'adjoint_inverse' key in the 'solver_options', just for future reference.

The tests currently seem to fail bc. of the newton, which might be related to 45c797f?