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. 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...

this PR must be merged synchronously with OPM/opm-core#688...
I don't see a call to this method (`averageTemperature()`) from anywhere in `SurfaceToReservoirVoidage`.

Won't `getRegTemp()` just return zero (or whatever is left in the memory area for `T_avg_` from earlier operations) for all regions then?  What about calls to `props_.rsSat()` and `props_.rvSat()` in `calcRmax()` that use `T_avg_` directly?

Is this a problem?