MetaD restart from GRID

[makedoc]

close #21

CHANGES/Unreleased.txt

@@ -46,5 +46,6 @@ Changes from version 2.2 which are relevant for users:
   - \ref COMMITTOR can now be used to define multiple basins
   - The number of atoms admitted in \ref BRIDGE has been significantly increased, see \issue{185}.
   - \ref driver now allows --trajectory-stride to be set to zero when reading with --ixtc/--itrr. In this case, step number is read from the trajectory file.
+  - MetaD and PBMetaD can now be restarted from a GRID 
 
 */

src/bias/MetaD.cpp

@@ -85,6 +85,10 @@ In case you do not provide any information about bin size (neither GRID_BIN nor
 and if Gaussian width is fixed PLUMED will use 1/5 of the Gaussian width as grid spacing.
 This default choice should be reasonable for most applications.
 
+Metadynamics can be restarted either from a HILLS file as well as from a GRID, in this second
+case one can first save a GRID using GRID_WFILE (and GRID_WSTRIDE) and at a later stage read
+it using GRID_RFILE.
+
 Another option that is available in plumed is well-tempered metadynamics \cite Barducci:2008. In this
 varient of metadynamics the heights of the Gaussian hills are rescaled at each step so the bias is now
 given by: