Improvements to local condensation of constraints

[fredrikekre]

Some changes extracted from #567, see commits and commit messages.

Perhaps addindex! should just be methods of assemble!, they do the same thing, really (with argument order reversed since addindex! matches Base.setindex!).

[codecov-commenter]

Codecov Report

Base: 92.88% // Head: 0.00% // Decreases project coverage by -92.88% ⚠️

Coverage data is based on head (e65224f) compared to base (4de2b6c).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #571       +/-   ##
==========================================
- Coverage   92.88%   0.00%   -92.89%     
==========================================
  Files          25      25               
  Lines        4064    4016       -48     
==========================================
- Hits         3775       0     -3775     
- Misses        289    4016     +3727     

Impacted Files	Coverage Δ
src/Dofs/ConstraintHandler.jl	0.00% <0.00%> (-95.93%)	⬇️
src/arrayutils.jl	0.00% <0.00%> (-65.63%)	⬇️
src/assembler.jl	0.00% <0.00%> (-99.33%)	⬇️
src/L2_projection.jl	0.00% <0.00%> (-100.00%)	⬇️
src/FEValues/cell_values.jl	0.00% <0.00%> (-100.00%)	⬇️
src/FEValues/face_values.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Grid/grid_generators.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Dofs/apply_analytical.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Quadrature/gaussquad_tet_table.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Quadrature/gaussquad_tri_table.jl	0.00% <0.00%> (-100.00%)	⬇️
... and 18 more

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