The previous text contained an ambiguity of the contents of the field
global_cell in case it was non-null; this could be interpreted as if
the grid was non-Cartesian. However, this is not the convention that
has been followed in current clients.
The new text writes a warning about this, as well as giving a tip on
how an unstructured grid may be signaled through the structure (the
cartdims field is zero-initialized in the create_empty_grid function,
so this array must be actively initialized by the client).
There is little to no error checking, and the importer assumes that the
grid is serialised more or less directly from the grid structure.
Intended use: Testing on non-uniform grids created in MRST.
This is mostly for use in *other* grid constructors, as it enables
putting a grid into a defined state before allocating and/or filling
the specific fields of the UnstructuredGrid.
While here, add a Doxygen \file command lest function documentation
not be output to the documentation set.
