Fix bug with L2projection of mixed grid

[lijas]

We were not using the local index to get the quadrature point data, so it was not working for arbitrary cellsets.

[codecov-commenter]

Codecov Report

Merging #456 (d58315a) into master (b46853e) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
- Coverage   91.50%   91.33%   -0.17%     
==========================================
  Files          22       22              
  Lines        3259     3405     +146     
==========================================
+ Hits         2982     3110     +128     
- Misses        277      295      +18     

Impacted Files	Coverage Δ
src/L2_projection.jl	100.00% <100.00%> (ø)
src/Grid/grid.jl	85.66% <0.00%> (-3.62%)	⬇️
src/assembler.jl	98.34% <0.00%> (ø)
src/Grid/coloring.jl	92.30% <0.00%> (ø)
src/Dofs/ConstraintHandler.jl	92.34% <0.00%> (ø)
src/Dofs/DofHandler.jl	86.81% <0.00%> (+3.29%)	⬆️

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[kimauth]

Lgtm. I did confuse myself with the fact that we call the vector of cell indices set while reviewing though, perhaps we should change that to something like cellindices or cellids.

[fredrikekre]

I did confuse myself with the fact that we call the vector of cell indices set while reviewing though

Same!