This commit introduces a fairly general mechanism for accessing the
active subset of a global grid (property) array. Essentially, this
takes on the role of translating the active cell index through the
"global_cell" mapping when accessing, e.g., the net-to-gross data
value.
The primary component is class template
Opm::GridPropertyAccess::Compressed<DataArray,Tag>
which implements a read-only
value_type operator[](const int c)
that encapsulates and performs the compressed-to-global cell index
translation. Template parameter "DataArray" is intended as a policy
parameter that for instance wraps access to a "GridProperty<T>" from
module opm-parser (with a fall-back default value if the data is not
specified on input). The "Tag" parameter is a provision for type
safety--e.g., to prevent passing a region ID into a function that
requires a porosity value.
