Document Cartesian info fields for unstructured grids

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).
Merge pull request #359 from rolk/359_cart

Document Cartesian info fields for unstructured grids
Merge pull request #352 from andlaus/refactor_exception_classes

Refactor exception handling code
import Dune's class name code
Import the property system from eWoms
add authors to the files according to git logs
add a test for the property system
make sure the header files related to the property system are installed