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...
I think that it would not be too much effort to have a non-isothermal equilibration process provided that the temperature is specified externally, e.g., using the geothermal gradient.

> however I fear that one could end up with trouble due to this standard temperature (being used as a dummy so far) being defined in multiple spots.

that's true, but I had some trouble understanding the design of the equilibration code and thus did not know where to put it centrally...