Implement multi-region PVT properties

this patch implements multi-region PVT support for all PVT property classes. It is based uppon #583, so this cannot be merged immediately without breaking opm-porsol and all modules which depend on it. If somebody wants to rebase this on the current master to make it compile with the old eclipse parser, I'll appreciate patches, but I won't do this work myself. (sorry, I'm quite determined for this point.)

I tried to test this patch as far as currently possible, i.e., no ctest regressions on my machine, `sim_fibo_ad` seems to work for SPE-1 and SPE-9 as before, and the Norne deck does get as far as without this patch (On Norne, there is still the problem with endpoint scaling, see OPM/opm-autodiff#129, but even with the patch applied and using `gwseg` as threephase_model, it still produces a lot of NaNs in the update of the first iteration).

also, this patch is _huge_, but I can be split up into smaller chunks if @alfbr gives me at least one week to do it...

Finally, this patch needs to be merged synchronously with the corresponding PR for opm-autodiff because autodiff's copy of `BlackoilPVTProps` needs to be adapted...