fix: disallow complex basis vectors for now

[gdalle]

• Only allow creation of basis arrays when the eltype is a subtype of Real
• Enforce that the pushforward fallback for derivative applied to Real inputs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.36%. Comparing base (9b17e3e) to head (aa4a360).

❗ There is a different number of reports uploaded between BASE (9b17e3e) and HEAD (aa4a360). Click for more details.

HEAD has 21 uploads less than BASE

Flag	BASE (9b17e3e)	HEAD (aa4a360)
DI	35	14

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #669       +/-   ##
===========================================
- Coverage   97.61%   81.36%   -16.25%     
===========================================
  Files         112      111        -1     
  Lines        5609     5586       -23     
===========================================
- Hits         5475     4545      -930     
- Misses        134     1041      +907     

Flag	Coverage Δ
DI	74.19% <100.00%> (-24.57%)	⬇️
DIT	95.35% <ø> (ø)

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