Move GridManager to grid subdir.

Also remove GridAdapter (moved to dune-cornerpoint), and
moved grid.c implementation file to grid subdir.
Moved deprecated opm/core/well.h -> opm/core/pressure/legacy_well.h

Still used by some pressure system assemblers.
Renamed newwells.h -> wells.h.

Also moved implementation file to subdir.
Further reorganising of opm-core.

Deleted some unused code (or moved to opm-porsol), moved all code dealing with
time-of-flight to opm/core/tof, moved code for implicit transport solver to
opm/core/transport/implicit, spu_[im|ex]plicit.[ch] to opm/core/transport/minimal.
Update to follow previous moves and deletes.