Reset internal force of point mass before every simulation step

[jslee02]

The soft body simulation behaves unexpectedly when the simulation option is resetCommand = false. The root cause was that the internal forces of point masses, which are used for soft body simulation, are not reset to zero in every simulation step. In the rigid body case, this issues doesn't happen because the internal forces are set to the control command. Soft body, however, doesn't have control command API so it keeps the previous value, and this leads to the unexpected behavior.

This PR fixes the issue by always setting the internal forces of point mass to zero regardless of the resetCommand option.

Resolves #1370

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[codecov]

Codecov Report

Merging #1372 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1372      +/-   ##
=========================================
+ Coverage   56.89%   56.9%   +<.01%     
=========================================
  Files         366     366              
  Lines       27389   27390       +1     
=========================================
+ Hits        15584   15585       +1     
  Misses      11805   11805

Impacted Files	Coverage Δ
dart/dynamics/SoftBodyNode.cpp	39.39% <100%> (+0.03%)	⬆️
dart/dynamics/PointMass.cpp	45.9% <100%> (ø)	⬆️