Implemented OFile::rewind
Modified behavior of grid file in METAD

When not RESTARTing:
file is backed up if existing.
  with STORE_GRIDS, file grows during simulation
  without STORE_GRIDS, file is replaced every time grid is rewritten

When RESTARTing:
  with STORE_GRIDS, file is appended and grows during simulation
  without STORE_GRIDS, file is replaced every time grid is rewritten

This hopefully fixes #34
Regtest for WRITE_GRID
Added flush and some comment

Without STORE_GRIDS it is necessary to flush the grid file,
otherwise it might stay empty for the entire simulation