Make the CellSimilarity argument const also in Mapping.

Make the CellSimilarity argument const also in Mapping.

Logically, these arguments are intended to be input arguments, so make them
const. Also, don't make them a reference as they are simply integral values.

That said, the original intention of having a reference was so that
mappings can indicate situations where cell similarity has to be
disregarded. Consequently, let the fill_fe_values() functions return a
cell similarity value that will then simply be assigned to the same
variable again.