Make the internal data object of the mapping classes const.

Make the internal data object of the mapping classes const.

Logically, they are input arguments to the fill_fe_values() functions, they are only
initialized in get_data() etc. Thus, make them const in the fill_fe_values functions.
There are places where mapping classes want to move data across from fill_fe_values
in terms of temporary arrays to the transform() function that may be called right
afterwards from FiniteElement::fill_fe_values(). In those cases, make these members
mutable.