fix: replace one with oneunit for basis computation

[gdalle]

For unitful quantities, the right way to construct a basis or initialize a derivative / gradient is with oneunit(T) instead of one(T) (the latter is just a multiplicative identity).

Fixes JuliaGeometry/MeshIntegrals.jl#191

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (ea73473) to head (6803e2f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
+ Coverage   97.88%   97.89%   +0.01%     
==========================================
  Files         129      129              
  Lines        7752     7751       -1     
==========================================
  Hits         7588     7588              
+ Misses        164      163       -1     

Flag	Coverage Δ
DI	98.73% <100.00%> (ø)
DIT	95.66% <100.00%> (+0.04%)	⬆️

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