our policy is that we only use boost if necessary, i.e., if the oldest
supported compiler does not support a given feature but boost
does. since we recently switched to GCC 4.4 or newer, std::shared_ptr
is available unconditionally.
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.