Add parallel matrix assembly using OpenMp

[pmueller2]

Adds parallelization of the assembly loops using OpenMp. CMake option ENABLE_OPENMP (default True) is added to turn it off. To change the default number of threads set the environment variable OMP_NUM_THREADS.

[codecov[bot]]

Codecov Report

Merging #257 into develop will increase coverage by 0.02%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #257      +/-   ##
===========================================
+ Coverage    90.83%   90.86%   +0.02%     
===========================================
  Files          326      326              
  Lines        10871    10898      +27     
===========================================
+ Hits          9875     9902      +27     
  Misses         996      996

Flag	Coverage Δ
#integrationtests	64.46% <70.58%> (+0.05%)	:arrow_up:
#unittests	96.24% <100%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
nuto/mechanics/cell/SimpleAssembler.h	100% <ø> (ø)	:arrow_up:
nuto/mechanics/cell/SimpleAssembler.cpp	100% <100%> (ø)	:arrow_up:
nuto/math/NaturalCoordinateMemoizer.h	83.33% <0%> (+0.98%)	:arrow_up:
nuto/base/Timer.cpp	91.17% <0%> (+2.71%)	:arrow_up:

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