Fix Mesh.flip(mask=None): Take care of the mask

[adtzlr]

fixes #624 (as mentioned in #622)

Example

from skfem import MeshTet

mesh_skfem = MeshTet().refined(2)
mesh_skfem.save("trialMesh.xdmf")

from meshio import xdmf
import felupe as fem
import numpy as np

meshio_mesh = xdmf.read("trialMesh.xdmf")
mesh = fem.Mesh(
    points=meshio_mesh.points,
    cells=meshio_mesh.get_cells_type("tetra"),
    cell_type=meshio_mesh.cells[0].type,
)

# ensure positive cell volumes (before adding midpoints on edges!)
region = fem.RegionTetra(mesh)
mesh = mesh.flip(np.any(region.dV < 0, axis=0))
region.reload(mesh)
assert np.all(region.dV > 0)

region_u = fem.RegionQuadraticTetra(mesh.add_midpoints_edges())
region_p = fem.RegionTetra(
    mesh=fem.mesh.dual(mesh, disconnect=False),
    quadrature=region_u.quadrature,
    grad=False,
)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cafae00) 96.30% compared to head (cac217b) 96.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #625   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files          91       91           
  Lines        4681     4681           
=======================================
  Hits         4508     4508           
  Misses        173      173           

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