Remove some unnecessary allocations in affine constraint condensation

[fredrikekre]

This patch removes some unnecessary allocations of a sparse matrix column which was needed before when the sparse matrix changed in-place. Since #436 we create a new matrix for the new entries and then add them in the end instead.

Comparison for the Stoke example in the docs:

• Master:

  julia> @time create_sparsity_pattern(dh, ch);
    0.009940 seconds (2.91 k allocations: 13.387 MiB)

• PR:

  julia> @time create_sparsity_pattern(dh, ch);
    0.004960 seconds (66 allocations: 12.581 MiB)

[codecov-commenter]

Codecov Report

Base: 92.56% // Head: 92.56% // No change to project coverage 👍

Coverage data is based on head (2ada10e) compared to base (c90c188).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #537   +/-   ##
=======================================
  Coverage   92.56%   92.56%           
=======================================
  Files          22       22           
  Lines        3901     3901           
=======================================
  Hits         3611     3611           
  Misses        290      290           

Impacted Files	Coverage Δ
src/Dofs/ConstraintHandler.jl	95.55% <100.00%> (ø)

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