Merge DofHandler and MixedDofHandler

[kimauth]

After a series of previous PRs, the MixedDofHandler is now equally performant as the DofHandler in all performance critical parts of the code base (see #629).

This PR removes DofHandler and those code paths that specialised for it. MixedDofHandler is renamed to DofHandler.

This should represent a non-breaking change for users using DofHandler. Users using MixedDofHandler need to rename all occurrences of MixedDofHandler to DofHandler, but their code should keep working otherwise.

closes #624

[fredrikekre]

Run git mv src/Dofs/MixedDofHandler.jl src/Dofs/DofHandler.jl too?

[codecov-commenter]

Codecov Report

Patch coverage: 94.52% and project coverage change: +0.14 🎉

Comparison is base (539e49e) 92.99% compared to head (fbc1b9b) 93.14%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #667      +/-   ##
==========================================
+ Coverage   92.99%   93.14%   +0.14%     
==========================================
  Files          30       29       -1     
  Lines        4499     4273     -226     
==========================================
- Hits         4184     3980     -204     
+ Misses        315      293      -22     

Impacted Files	Coverage Δ
ext/FerriteBlockArrays.jl	86.66% <ø> (ø)
ext/FerriteMetis.jl	100.00% <ø> (ø)
src/Ferrite.jl	100.00% <ø> (ø)
src/Grid/grid.jl	90.36% <ø> (-0.53%)	⬇️
src/Dofs/DofHandler.jl	94.19% <94.02%> (+10.04%)	⬆️
src/Dofs/ConstraintHandler.jl	94.79% <100.00%> (-1.05%)	⬇️
src/Dofs/DofRenumbering.jl	91.39% <100.00%> (-0.10%)	⬇️
src/Dofs/apply_analytical.jl	100.00% <100.00%> (+1.81%)	⬆️
src/Dofs/sparsity_pattern.jl	99.27% <100.00%> (ø)
src/L2_projection.jl	100.00% <100.00%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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[fredrikekre]

Ferrite.jl/ext/FerriteMetis.jl

Line 32 in b2b86dd

dh::Union{DofHandler,MixedDofHandler},

[fredrikekre]

Thanks @kimauth for the herculean effort!