Enhances usability of interpolate() with BoundaryUID argument

The commit adds a member function

  interpolate( ValueType constant, uint_t level, BoundaryUID boundaryUID )

to the following functions:

 - P1Function a.k.a. VertexDoFFunction
 - EdgeDoFFunction
 - P2Function
 - CSFVectorFunction

Additionally we also add the following two variants to CSFVectorFunction,
which allow to interpolate the components by different constants:

 interpolate( const std::vector< valueType >&, size_t, DoFType )
 interpolate( const std::vector< valueType >&, size_t, BoundaryUID )