Add API to get mesh edge lengths (and others)

[veenstrajelmer]

To support easy visualisation of mesh properties. The current exposed properties are these:

Also: there is a mk.mesh2d_delete_small_flow_edges_and_small_triangles() available, but this is a combination of two actions and it is not possible to check if it is needed or not. Check DCSM workflow for more. Interacter defaults: small-links=0.1 (which means 0.1*sqrt(baL+baR)), small-triangles=0.2 (triangle removed if triaarea < average adjacent areas). Latter is often not used.