Re-add getfieldnames(::FieldHandler)

[KnutAM]

xref #647 and #648

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.03 ⚠️

Comparison is base (a350434) 92.99% compared to head (57fe9bb) 92.97%.

❗ Current head 57fe9bb differs from pull request most recent head 1706b05. Consider uploading reports for the commit 1706b05 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   92.99%   92.97%   -0.03%     
==========================================
  Files          30       30              
  Lines        4496     4497       +1     
==========================================
  Hits         4181     4181              
- Misses        315      316       +1     

Impacted Files	Coverage Δ
src/Dofs/MixedDofHandler.jl	91.43% <0.00%> (-0.32%)	⬇️

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