Assertion string and typo fix

This is kind of a "Hello" merge request, since I am intending to work on GROMACS in the next months to develop a post-processing tool to extract the individual forces acting on the particles (opposed to the total force). If there is interest, it is possible to upstream this changes.

Any guidance for what the best approach would be or where to start is much appreciated.

While getting comfortable with the codebase I noticed these slip throughs.

Fixes #3597 (closed)

Best regards, Simon Christ