- Plugin(Skin) now works on adapted datasets
- addData() should now also work for high order datasets [we still need to
  add a new type+ref to FS in PViewDataGModel]

- new GetEnv and GetString functions

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- work on plugin user-friendliness (short help & co)

in a force field (should be generalized e.g. for Lorentz force,
but that a TODO ;-)

Some code cleanup & added comments.

- invert #define logic (HAVE_MESH, HAVE_POST, HAVE_SOLVER, ... instead of HAVE_NO_MESH,
  HAVE_NO_POST, HAVE_NO_SOLVER)