Update Colvars library to version "2023-12-04"

This change contains an update of the Colvars library in src/external (in a single commit) and small edits to GROMACS files (in separate commits for easier review).

The GROMACS commits address the following issues:

• MSVC compilation warnings (see #4924 (closed))
• Propagate OpenMP build flags to the library when requested for GROMACS
• Disable initialization to zero of the initial step number for Colvars, allowing it to restart itself from the cpt file. Not supported yet is the case where init-step is set to a non-zero value in the MDP (i.e. the Colvars internal step counter will always start at zero, unless when reading a checkpoint).
• Track the version of the GROMACS-Colvars interface.

The message of the single commit for the Colvars library contains links to pull requests in the Colvars repo. A notable feature is the keyword defaultInputStateFile to allow loading previous Colvars data from text file in the few cases where a cpt cannot be read. If a CPT is read, that keyword is ignored to avoid inconsistencies in the expected input.