Add axisymmetric argument to FieldsMixed

[adtzlr]

Also fix wrong are normal of boundary regions.

Fixes #200

[codecov-commenter]

Codecov Report

Merging #199 (223de8b) into main (3a623df) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
- Coverage   98.32%   98.31%   -0.01%     
==========================================
  Files          62       62              
  Lines        2504     2499       -5     
==========================================
- Hits         2462     2457       -5     
  Misses         42       42              

Impacted Files	Coverage Δ
felupe/_assembly/_axi.py	100.00% <100.00%> (ø)
felupe/_field/_axi.py	95.45% <100.00%> (+1.01%)	⬆️
felupe/_field/_fields.py	100.00% <100.00%> (ø)
felupe/quadrature/_gausslegendre.py	100.00% <100.00%> (ø)
felupe/region/_boundary.py	100.00% <100.00%> (ø)
felupe/region/_templates.py	100.00% <100.00%> (ø)

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