Add facility for accessing active subset of global data array

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.

tests/compressed_gridproperty.data

@@ -0,0 +1,48 @@
+RUNSPEC
+
+DIMENS
+  1 1 4
+/
+
+GRID
+
+COORD
+  0 0 0  0 0 0
+  1 0 0  1 0 0
+  0 1 0  0 1 0
+  1 1 0  1 1 0
+/
+
+ZCORN
+  0 0 0 0
+  1 1 1 1
+
+  1 1 1 1
+  2 2 2 2
+
+  2 2 2 2
+  3 3 3 3
+
+  3 3 3 3
+  4 4 4 4
+/
+
+ACTNUM
+  0 1 0 1
+/
+
+NTG
+  0.1 0.2 0.3 0.4
+/
+
+REGIONS
+
+SATNUM
+  4 3 2 1
+/
+
+-- Note: Dummy 'PROPS' and 'SCHEDULE' sections only
+PROPS
+SCHEDULE
+
+END