Add Volumetric, CompositeMaterial and &-operator for ConstitutiveMaterial

[adtzlr]

Example

import felupe as fem

nh = fem.NeoHooke(mu=1.0)
vol = fem.Volumetric(bulk=2.0)
umat = nh & vol
ax = umat.plot()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ade27e2) 97.11% compared to head (cf35b77) 96.30%.
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
- Coverage   97.11%   96.30%   -0.82%     
==========================================
  Files          91       91              
  Lines        4652     4681      +29     
==========================================
- Hits         4518     4508      -10     
- Misses        134      173      +39     

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