Add Odeint<->Eigen helper file + small example

[Psirus]

see above

[codecov[bot]]

Codecov Report

Merging #248 into PDE_reviewed will increase coverage by <.01%. The diff coverage is 87.5%.

@@               Coverage Diff                @@
##           PDE_reviewed     #248      +/-   ##
================================================
+ Coverage         84.41%   84.41%   +<.01%     
================================================
  Files               294      296       +2     
  Lines             10828    10860      +32     
================================================
+ Hits               9140     9168      +28     
- Misses             1688     1692       +4

Flag	Coverage Δ
#integrationtests	62.02% <87.5%> (+0.11%)	:arrow_up:
#unittests	87.69% <ø> (ø)	:arrow_up:

Impacted Files	Coverage Δ
nuto/math/EigenOdeintCompatibility.h	0% <0%> (ø)
...tions/integrationtests/math/HarmonicOscillator.cpp	100% <100%> (ø)

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[Psirus]

Muhaha, now this grave mistake will be set in stone for all eternity