Added GRID_SPACING to METAD + automatic grid settings

It is now possible to specify GRID_SPACING instead of GRID_BIN.
If both are specified, the most conservative choice is used for each CV.
If none are specified, 1/5 of hills width is used (this is
very conservative but will avoid mistakes).
Added regtest for METAD with new grid options
Changelog for grid spacing stuff
Merge remote-tracking branch 'origin/master'