Severs the code dependency on opm-commmon. There was no actual
functional dependency here, with the exception of some enable/disable
warning headers. To properly make opm-parser a stand-alone module the
usage of these headers have been removed and the dependency on
opm-common is gone.
This is an effort to improve build performance. Several includes
scattered across the project are either unused or partially used (i.e.
just used to import a type name, not depending on the actual contents of
the header file).
Replaces a lot of these includes with forward declarations.
