- METAD: exit with error if used
- PBMETAD: gives a warning and deactivate the interval for the specific variable
close #181

Added an option in OFile to enforce backup of files even if restarting

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close #21

- added the restart from grid to PBMETAD
- more error checking

- 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

(courtesy of Andrea Cesari)

Made it so that you can inherit from DRMSD to make it easy to make new combinations of DRSMD distances

I think this fixes #143

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fix a problem when md.log was appended
plumed was trying to write on it after
md.log was already closed