METAD: changed the behavior of restart from GRID

- when restart from GRID (RGRID+RESTART) doens't read the HILLS (but continue to append)
- if not restart then the HILLS are added to the existing GRID
- the grid is not external anymore, in this way when you write the grid you write the total one
- grid files are written only once in case of multiple walkers
NOTE: the grid is not modified for the BIASFACTOR because the BIASFACTOR is not currently stored
in the grid, this means that if you restart from a grid