[makedoc]

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

I cleaned up some of the (innocuous) warnings

this check should be done also in NDEBUG mode

Same as #171 but using C++ standard include file <ctime>

Closes #171

This bug was introduced by me in 5e3aaf3a
The barrier on multi replicas was due to overcaution and
is only necessary when using WALKERS_MPI

It seems that sometime on travis a regtest with mw-mpi crashes.

I think this is because when using multiple processors per walker
there is a problem in the opening of the old file (for read) and
its backup (for write). Adding this barrier seems to make it more robust

Related to: e1b845b4

Now the type is defined with a typedef so that it is easier to change it.

I also set the type to size_t instead of unsigned long long.
This should be more portable.

Only add to the manual RESTART UPDATE_FROM and UPDATE_UNTIL
in the actions for which this makes sense

Allow individual action to set restart, using RESTART=YES or
RESTART=NO. By default, RESTART=AUTO, that means that global
restart settings are used.

Notice that this required to change the way OFile knows about
restart. Indeed, if the file is linked to an action,
it should use Action::getRestart instead of PlumedMain::getRestart.