Fix build in !HAVE_DUNE_CORNERPOINT case

This change-set is a roll-up of the modifications I had to do to make the "refactor-for-cpgrid-support" branch (cf. OPM/opm-core#518) build *without* dune-cornerpoint.  The most important commit is 89f282f.  Without that the build fails with linker errors.  The other changes are perhaps more cosmetic (although 4fe22ea does correct a semantic problem in the "TransTpfa_impl.hpp" header).

To be honest, I don't quite understand why 1862476 was needed.  This could well be a compiler bug.  In any case, declaring a function is hardly a problem.