Porv

With this PR the EclipseState object will calculate the PORV value. The implementation is based on adding a post processor to the GridProperty objects, in addition to PORV calculation a post processor has been added to  copy the PERM? and PORO values from the top layer to the lower layer - if they have not been fully specified in the input deck. The post processor correctly handles `MULTPV` and explicit `PORV` values in the deck.

Calling scope should get the PORV property with: 
```
EclipseState::getDoubleGridProperty("PORV")
```

There were some problems with the unit tests: see the comment in BoxTest.cpp at e.g. https://github.com/joakim-hove/opm-parser/commit/b05e5664a2699272f5b9c9773d4c368ffd332f37 - any fixes to this weirdness highly appreciated.