VectorFunction enhancements

Hi,

this issue is based on the comments for !381 (merged). That merge enhanced the implementation and functionality of the P1VectorFunction class. My comments were:

• Maybe we can move some of the methods (e.g. add, assign, dotLocal,...) upwards to a common base class instead of re-implementing basically the same stuff again in P2VectorFunction. But that is under discussion anyhow. Question is, if we get this to work nicely together with the templatisations.
• In that respect maybe also filter could be a free function?
• I definitely suggest to rename getMaxMagnitude() for VectorFunctions, as IMHO the name is very misleading. If I ask for the maximal magnitude of a velocity field e.g. I'd expect to get that back and not the maximal magnitude over all components.

Cheers
Marcus