This is making substantial changes to the Poisson integration code, and this is creating many conflicts with recent changes in the grid code.

Incidentally, colvargrid.cpp is way too long, largely because of the complex integrate_potential class. I'm thinking it would be better to split it off into its own file first in master, and then squash and merge this PR so that there is less interference with the colvar_grid classes.

What do you think @giacomofiorin ?

I'm okay with that; colvargrid.cpp is probably even larger than it looks with all the template instantiations.

The commit doing the split is now in the branch. The rest of the changes will mostly happen in the new files colvargrid_integrate.{cpp,h}.

If any work on colvargrid has to happen before this branch is merged, then the first commit should be cherry-picked first to prevent conflicts.