Change the parsing of functions in muparser.

muparser requires that there be no space between the name of a function and its argument
list. This is incompatible to the behavior we had with fparser that allowed for spaces
in this position. To allow for backward compatible behavior, this patch changes the
parsing in such a way that it eats spaces after the function name for all built-in
and deal.II-defined functions before passing the expression to muparser.

This addresses bug #218 at https://code.google.com/p/dealii/issues/detail?id=218 .