Move away from re2/rea internal mesh zones

[vbaconnet]

This PR gets rid of the internal BCs from the re2/rea formats. The idea is that the "classic" labels w,v,o,on,sym,shl (and its variations) are transformed into labeled zones with indices 1...6.

For example, if the mesh contains boundaries with W, v and on, we will now have to specify "boundary_types": ["w", "v","","","on"].

Note that if one has several boundaries set internally as one of those re2 labels, they will all be marked in the same labeled zone. For example, if I have a channel with a lower and upper wall they will both be marked as labeled zone 1, and one will not be able to modify them separately (say, if I want to top wall to become an open boundary instead).

TODO before undrafting:

• [ ] Do the same logic with rea files

NOTE: this has not been tested with meshes that have both internal labeled zones AND internal re2 labels like v,w,o etc.