Messages related to NOPBC keyword were wrong

Thanks to @valsson

- added some info in the error message of the parser
- std::string everywhere

Includes mention to ABIN code

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