Speed-boost for close!(dh::MixedDofHandler)

[kimauth]

Edges out a couple of anti-patterns in close!(dh::MixedDofHandler).

Example benchmark:

function construct_mixed_dof_handler(grid)
    mixed_dh = MixedDofHandler(grid)
    push!(mixed_dh, :f1, 1, Lagrange{2,RefCube,1}())
    push!(mixed_dh, :f2, 2, Lagrange{2,RefCube,1}())
    push!(mixed_dh, :f3, 1, Lagrange{2,RefCube,2}())
    close!(mixed_dh);
end

small_grid = generate_grid(Quadrilateral, (10, 10))
@btime construct_mixed_dof_handler($small_grid); 

# master: 196.791 μs (4493 allocations: 324.43 KiB)
# this PR: 52.959 μs (153 allocations: 186.04 KiB)

or for a bigger grid:

big_grid = generate_grid(Quadrilateral, (1000, 1000))
@btime construct_mixed_dof_handler($big_grid); 

# master: 3.821 s (70005918 allocations: 2.69 GiB)
# this PR: 1.229 s (454 allocations: 1.05 GiB)

Sidenote: Unfortunately the MixedDofHandler is still twice as slow as the DofHandler even with this PR, for comparison:

function construct_dof_handler(grid)
    dh = DofHandler(grid)
    push!(dh, :f1, 1, Lagrange{2,RefCube,1}())
    push!(dh, :f2, 2, Lagrange{2,RefCube,1}())
    push!(dh, :f3, 1, Lagrange{2,RefCube,2}())
    close!(dh)
end
@btime construct_dof_handler($small_grid) # 28.209 μs (140 allocations: 119.84 KiB)
@btime construct_dof_handler($big_grid) # 751.856 ms (341 allocations: 630.22 MiB)

[codecov-commenter]

Codecov Report

Base: 92.31% // Head: 92.31% // No change to project coverage 👍

Coverage data is based on head (82b60c8) compared to base (8e447e2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #533   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files          22       22           
  Lines        3776     3776           
=======================================
  Hits         3486     3486           
  Misses        290      290           

Impacted Files	Coverage Δ
src/Dofs/MixedDofHandler.jl	86.29% <100.00%> (ø)

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