broken user interface for VolumeConstraints in constraints.hh

In file dune/pdelab/constraints/common/constraints.hh in struct VolumeConstraints please remove the line "CL cl;" within leaf(...), so that the user-implemented Volume constraints effecting the member variable cl initialized in the constructor can take effect. Otherwise that line overwrites any Volume constraints. Bug seems to be present in both pdelab 2.5/2.6