Pass optional variable allow_deactive_cells=true to grid constructor

[joakim-hove]

The Downstream of OPM/opm-common#1059 and OPM/opm-grid#393

The grid construction process employed in opm/flow is a three step process:

1. Create initial grid with EclipseGrid(deck)
2. Load all 3D properties in the EclipseState
3. Inspect the PORV property derived from the EclipseStateand deactivate cells with small pore volume.

When the grid is constructed in this way the distribution of active cells should be correct already in the EclipseGrid, and the processing to create a Dune::cpGrid should not alter the set of (active) cells. With the PR set originating in OPM/opm-common#1059 we exploit this to do some simplifications, and furthermore add a hard check in opm-grid:processEclipseFormat() that no cells have been deactivated. Unfortunately the grid construction in opm-upscaling only contains the first step above, and the grid processing in opm-grid can disable additional cells. This PR passes a new allow_deactive_cells=true flag to the processEclipseFormat() function.

Yes - I know it is ugly :-(