Crossings are now not editable.
Edge flip is not used to handle topology changes + clean up.
Changed the collapse functions such that they resemble the ones in 3D DSC.

The collapses are performed to maximise the face quality, interface
edges can be collapsed if the neighbourhood is sufficiently flat and
the nodes can now not move outside the design domain. TODO: The
collapse function can still not handle boundary edges.
Made fields in the VelocityFunc class protected.