fix LangevinSplitting

[noeblassel]

Fixes a bug in Langevin Splitting. For certain splittings (such as "BAOAB"), the first "B" step will not recompute forces for the sake of efficiency. Before this PR, the forces were initialized via asimilar call, which may result in NaNs propagating during the integration steps.

The PR fixes this by precomputing the forces once before the integration takes place.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.69%. Comparing base (7b1f972) to head (05766b2).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
+ Coverage   67.41%   67.69%   +0.27%     
==========================================
  Files          37       38       +1     
  Lines        5678     5686       +8     
==========================================
+ Hits         3828     3849      +21     
+ Misses       1850     1837      -13     

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[jgreener64]

Thanks for spotting. Could you move it up to where forces_nounits_t is defined, like in NoseHoover?

I will make a release with this and any other fixes before the summer school.

[noeblassel]

Thanks a lot. It's done.