Implement the copy constructor for DiscreteFunction

Praetorius, Simonrequested to merge
issue/discretefunction_copy_constructor into master

This solves a bug in LocalFunction and GradientLocalFunction of DiscreteFunction. If copy constructed, the subTree member was a dangling pointer.

Maybe I have to add the copy-assignment operator as well. Or, better, a swap() method.

Edited by Praetorius, Simon

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