Marker point nodes become invalid when mesh changes

[rchristie]

User defined marker points by default get the next node numbers after those used by the model.

However in box network scaffold changes to the lengths of segments or element density or change of algorithm can increase the number of nodes in the model so model node numbers are referenced by existing marker points.

This can lead to exceptions.

Need a technique for automatically increasing marker node numbers to avoid these clashes.

Similar issue happen when network layout annotations change, or there are annotation-specific numbers of elements.