PVT properties: allow them to be temperature dependent

Note that this patch does not introduce any real temperature
dependence but only changes the APIs for the viscosity and for the
density related methods. Also note, that I also don't like the fact that
this requires so many changes to so many files, but with the current
design of the property classes I cannot see a way to avoid this...

the testing don is a bit limited so far: it compiles on clang 3.5, gcc 4.9 and gcc 4.4, and all ctests pass (whatever the latter means). If you have more tests, please run them before merging.

Finally, opm-autodiff and opm-polymer require a few changes which must be merged synchronously with this one...
This doc comment is misleading, as we do not yet provide the dbdT output.