Snapping back to the coarse grid

[UZerbinati]

We added a feature that allows you to snap the mesh in the hierarchy to the coarsest curved geometry, hence creating a nested mesh hierarchy. Example on how to use the code:

from firedrake import *
from netgen.occ import *
from netgen.meshing import MeshingParameters

disk1 = Circle((0, 0), 1).Face()
disk1.name = "bnd"
gelato = (disk1*HalfSpace((0,0,0),(0.2,0.6,0)))*HalfSpace((0,0,0),(0.2,-0.6,0))
geo = OCCGeometry(gelato, dim=2)
mp = MeshingParameters(minh=1.0)
ngmesh = geo.GenerateMesh(maxh=3.0, mp=mp)
mesh = Mesh(ngmesh)
mh = MeshHierarchy(mesh, 2, netgen_flags={"snap_to": "coarse", "snap_smoothing": "hyperelastic", "degree": 3})
for i in range(3):
    File(f"gelato_{i}.pvd").write(mh[i])