Attempt to implement SolidBodyNearlyIncompressibleX

[adtzlr]

• this works for disconnected dual meshes
• but not for connected dual meshes

I guess it's because the volume-matrices are not invertible for connected meshes.

I'm not sure if it's worth the effort for disconnected meshes only...

[codecov]

Codecov Report

❌ Patch coverage is 25.85034% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.91%. Comparing base (3ecf1a3) to head (f64797b).
⚠️ Report is 1488 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/felupe/mechanics/_solidbody_incompressible_x.py	17.34%	81 Missing ⚠️
src/felupe/mechanics/_helpers.py	25.00%	27 Missing ⚠️
src/felupe/field/_dual.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #640      +/-   ##
==========================================
- Coverage   97.07%   94.91%   -2.17%     
==========================================
  Files          94       95       +1     
  Lines        4752     4896     +144     
==========================================
+ Hits         4613     4647      +34     
- Misses        139      249     +110     

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