'work' is a small, constant size array for temporary data so it can be
allocated on the stack and the 'work' parameter can be eliminated entirely.
Thanks to Bård Skaflestad for the suggestion!
most of them quite insignificant, but still annoying. The only
exception is the warning about the changed alignment for the 'work'
argument of spu_implicit_assemble(). AFAICT, the only reason why it
worked was that the pointer produced by malloc() was passed
directly. (malloc() seems to fulfill all alignment criteria.) To fix
this, I've changed that argument's type from char* to double*.
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.
