warning on periodicity for grids is now an error becasue the grid spacing can be completely wrong

- added default value for r_0
- simplified examples

Exploits 07552b60

Added documentation on --enable-rpath

It now takes paths into account correctly

This makes it possible to use the command

port test plumed

to compile and test plumed.

Notice that tests on travis are still performed explicitly,
I plan to fix this in the near future, after having verified that there
are no problems with timings.

Minor changes to VES manual

cc @valsson

See https://groups.google.com/d/msgid/plumed-users/8fcca53b-38d1-470e-a413-a4339702c6db%40googlegroups.com

Thanks to Qingh Liao

cc @HanatoK I fixed the problem. Namely, plumed was linking by mistake
another boost version (homebrew).

HomeBrew boost was linked by mistake.

osx8.2 is deprecated.

I think it is better NOT to recommend sourcing "sourceme.sh". Most users
should install plumed with the canonical procedure (make install)

Notice that this breaks previous "check" target, that I think
was anyway useless.

check: does the check with the non-installed plumed
installedcheck: does the check with the installed plumed