In a parallel run each process only knows a part of the grid. Nevertheless
it does hold the complete well information. To resolve this the WellsManager
must be able to handle this case.
With this commit its constructor gets a flag indicating whether this is
a parallel run. If it is, then it does not throw if a well has cells that
are not present on the local part of the grid. Nevertheless it will check
that either all or none of the cells of a well are stored in the local part
of the grid.
Wells with no perforated cells on the local will still be present but set to SHUT.
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