Fix FieldsMixed() for MINI-elements

[adtzlr]

don't disconnect the dual mesh.

Description: Fix nearly-incompressible hyperelasticity on (bubble-enriched MINI) triangle/tetrahedrons.

Fixes #537

import felupe as fem
import pypardiso

mesh = fem.Cube(n=4).triangulate().add_midpoints_volumes(cell_type="tetra")
region = fem.RegionTetraMINI(mesh)
field = fem.FieldsMixed(region, n=3)

boundaries, loadcase = fem.dof.uniaxial(field, clamped=True)

umat = fem.ThreeFieldVariation(fem.NeoHooke(mu=1, bulk=5000))
solid = fem.SolidBody(umat, field)

move = fem.math.linsteps([0, 1], num=21)
step = fem.Step(items=[solid], ramp={boundaries["move"]: move}, boundaries=boundaries)

job = fem.CharacteristicCurve(steps=[step], boundary=boundaries["move"])
job.evaluate(tol=1e-3, solver=pypardiso.spsolve)
fig, ax = job.plot(
    xlabel="Displacement $u$ in mm $\longrightarrow$",
    ylabel="Normal Force $F$ in N $\longrightarrow$",
)

field.plot("Principal Values of Logarithmic Strain").show()

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f048961) 97.95% compared to head (590e603) 97.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #538   +/-   ##
=======================================
  Coverage   97.95%   97.95%           
=======================================
  Files          85       85           
  Lines        4110     4114    +4     
=======================================
+ Hits         4026     4030    +4     
  Misses         84       84           

Files Changed	Coverage Δ
src/felupe/_field/_fields.py	100.00% <100.00%> (ø)
src/felupe/mesh/_convert.py	96.46% <100.00%> (+0.09%)	⬆️
src/felupe/mesh/_mesh.py	98.92% <100.00%> (ø)

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