Updates DGFunction class

This adds a multElementwise() method to the DGFunction class and extends
the associated FunctionMultElementwiseTest. For this to work smoothly the
commit also performs the following two updates on the DGFunction:

- it adds an assign() version for a constant value
- it changes the interface of the other assign() method for functions
  to use a vector of reference wrappers instead of a vector of pointers
  so that it is consistent with assign in the other function classes,
  like P1Function etc.
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