fix a few harmless CLang warnings

i.e. rename class to struct in two forward declarations and remove an
unused private attribute...
adapt to the SimulatorTimer renames of opm-core
Merge pull request #53 from andlaus/compile_fixes

Compile fixes
remove forgotten reference to unused attribute "_gravity"

for some reason Clang compiled last time. I've got no idea why...
fix all remaining CLang warnings

this time, I've rebuild the whole module and it seems to work...
Merge pull request #54 from andlaus/more_compile_fixes

More compile fixes