Added possibility to specify kt in driver

I noticed that cache is not saved if brew is removed
(apparently some travis tool requires a brew library)

ERMSD now gives the same results irrespectively of units.

The value of PLUMED_ROOT stored in statically patched MD code was
incorrectly set to the compilation directory rather than the installed one.

The only way to trigger the bug from regtest was by observing the behavior
of rt-make-exceptions when plumed was installed by macports (since in this
case the compilation directory is removed).

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