Merge force and shellFC elements (modular simulator)
In the previous implementation, ForceElement was handling the call to do_force, while ShellFCElement was handling the call to relax_shell_flexcon. This lead to two nearly identical elements. The current change merges the two elements minimizing code duplication.
Rebased from: https://gerrit.gromacs.org/c/gromacs/+/15779
Edited by Pascal Merz