Fix tabulated potential

[christophlohrmann]

Fixes Issue discussed in ES-meeting (email by david power)

Description of changes:

• repulsive force now pushes particles away from each other

[codecov]

Codecov Report

Merging #2519 into python will increase coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           python   #2519    +/-   ##
=======================================
+ Coverage      72%     72%   +<1%     
=======================================
  Files         486     486            
  Lines       29447   29447            
=======================================
+ Hits        21341   21342     +1     
+ Misses       8106    8105     -1

Impacted Files	Coverage Δ
src/core/bonded_interactions/bonded_tab.hpp	12% <100%> (ø)	⬆️
src/core/nonbonded_interactions/nonbonded_tab.hpp	100% <100%> (ø)	⬆️
src/core/reaction_ensemble.hpp	68% <0%> (+2%)	⬆️

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