Space out 2D BLs

This new feature attempts to space out BL nodes (those projected from the boundary curves) in order to better fit the local, inner-domain target Delta. The feature is hidden behind a switch called SpaceOutBndLayer that corresponds to a threshold (of the ratio of the actual and the target Deltas) under correction is needed. If 2 nodes have a spacing that falls under that threshold, those nodes will be relocated. Sets are created of contiguous to-move nodes and each set is then expanded by one node on each side of the mesh (in order to find extra space for expansion). Nodes inside each set are then relocated according to a linear interpolation between the 2 set-end nodes using the local target Deltas. This is done iteratively under the threshold respected everywhere. This also has the effect of re-linearising the edge of the BL.

An additional parameter is provided as NoSpaceOutSurf that takes surfaces as value. This is simply a list of curves whose projected BL nodes should not be moved.

I reckon the name of the new parameters may not be perfect so I'm open to some renaming.

This is an example of what this new feature is able to achieve:

The feature was requested by @eec113. She has already tested it and is happy with the current results. She was able to generate satisfying meshes that she will now try to run them through the incompressible solver.

This branch also already includes !877 (merged) since the former comes as an expansion of the latter for this kind of very specific geometry.

I will also add a documentation entry later on as part of #82 (closed).