Merge branch 'feature/document-codim-parameter' into 'master'

document sementics of codim parameter for IndexSet::subIndex

If an entity of codimension cc > 0 is passed into IndexSet::subIndex, what
is the semantics of the 'codim' parameter? In DUNE, codimensions are always
with respect to the grid dimension (see method subEntity of reference element).

This patch clearly states that codim is to be understood with respect to the
grid dimension.

See merge request !97

dune/grid/common/indexidset.hh

@@ -141,7 +141,9 @@ namespace Dune
      *  \param[in]  e      reference to codimension cc entity
      *  \param[in]  i      number subentity of e within the codimension
      *  \param[in]  codim  codimension of the subentity we're interested in
-     *                     (must satisfy cc <= codim <= dimension)
+     *
+     *  \note The parameter <tt>codim</tt> denotes the codimension with respect
+     *        to the grid, i.e., it must satisfy cc <= codim <= dimension.
      *
      *  \return An index in the range 0 ... Max number of entities in set - 1.
      */
@@ -169,6 +171,9 @@ namespace Dune
      *  \param[in]  i      number subentity of e within the codimension
      *  \param[in]  codim  codimension of the subentity we're interested in
      *
+     *  \note The parameter <tt>codim</tt> denotes the codimension with respect
+     *        to the grid, i.e., it must satisfy cc <= codim <= dimension.
+     *
      *  \return An index in the range 0 ... Max number of entities in set - 1.
      */
     template< class Entity >