Expand unit test of InterpolationCompanion

[Psirus]

... and a couple of other small test improvements in /math.

[codecov[bot]]

Codecov Report

Merging #262 into develop will increase coverage by 0.39%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #262      +/-   ##
===========================================
+ Coverage    90.44%   90.83%   +0.39%     
===========================================
  Files          326      326              
  Lines        10843    10871      +28     
===========================================
+ Hits          9807     9875      +68     
+ Misses        1036      996      -40

Flag	Coverage Δ
#integrationtests	64.4% <ø> (+0.01%)	:arrow_up:
#unittests	96.24% <100%> (+0.51%)	:arrow_up:

Impacted Files	Coverage Δ
nuto/math/PolynomialLeastSquaresFitting.cpp	84.74% <ø> (+2.77%)	:arrow_up:
nuto/math/PolynomialLeastSquaresFitting.h	100% <ø> (ø)	:arrow_up:
test/math/EigenIO.cpp	100% <100%> (ø)	:arrow_up:
test/math/LinearInterpolation.cpp	100% <100%> (ø)	:arrow_up:
test/math/CubicSplineInterpolation.cpp	100% <100%> (ø)	:arrow_up:
...mechanics/interpolation/InterpolationCompanion.cpp	100% <100%> (ø)	:arrow_up:
test/math/EigenSparseSolve.cpp	100% <100%> (ø)	:arrow_up:
nuto/math/EigenSparseSolve.cpp	94.11% <0%> (+2.94%)	:arrow_up:
nuto/math/EigenIO.cpp	100% <0%> (+4.34%)	:arrow_up:
nuto/math/LinearInterpolation.cpp	100% <0%> (+7.14%)	:arrow_up:
... and 4 more

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