Mesh size fields

[simbilod]

Previous versions of this code had mesh size from a physical

Right now in here we only support constant characteristic length size according to the label name, see https://github.com/simbilod/meshwell/blob/main/meshwell/refinement.py

It would be nice to generalize to more types of resolutions settings

@nikosavola

[simbilod]

Implementation: dict of dicts of form:

physical name: { "resolution": x, "distance": y, "slope": z }

where:

• x is the constant resolution within the entities corresponding to physical tag
• y is a distance parameter for gmsh distance size field (how far until we recover default mesh size)
• z is a slope parameter for gmsh size field (more contorl over dropoff)

physical name should map to either "volumes" (N d) or "boundaries" (N-1 d); we already can tag boundaries, so should be able to map boundary physical names to dimtags for inputting into the size field

[simbilod]

closed as per https://github.com/simbilod/meshwell/pull/21