Resolve conflicts in extended ebMeta code

[giacomofiorin]

The code for the extended ebMeta method in this paper is currently in the branch below:
https://github.com/Colvars/colvars/tree/extended_ebmeta

Given that the version of Colvars is from late 2017, it cannot be used with versions of NAMD or LAMMPS that are currently supported. A GROMACS interface did not exist at the time.

Adding a PR would not be practical, therefore I am just adding an issue instead.

To use that feature in production, the commits in the branch should be broken down in smaller chunks and added back onto the current master.