Refactor getDisplacementFunction

[tarun-mitruka]

This PR was split from #221. See #221 for some PR review comments.

!!! UPDATE
std::optional version in finite elements has to be removed and the if-else branch in localSolutionBlockVector has to be determined during compile time. This will be addressed in #228

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2dfc766) 92.87% compared to head (a216aef) 92.73%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
- Coverage   92.87%   92.73%   -0.15%     
==========================================
  Files          49       50       +1     
  Lines        2022     1982      -40     
==========================================
- Hits         1878     1838      -40     
  Misses        144      144              

Flag	Coverage Δ
tests	92.73% <100.00%> (-0.15%)	⬇️

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[rath3t]

See comments

[rath3t]

Can you look into these minor things.
Than we can merge. Thanks Tarun!