Added MPI based implementation of multiple walkers

It is still preliminary and not very well optimized.
It only works with MPI (i.e. all processes should be part of the same
MPI job). In the future we might think to allow groups of MPI walkers
exchanging information through the filesystems.

Addresses issue plumed/plumed2#12
Added regtest for WALKERS_MPI
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
updated changelog
Merge branch 'v2.0'
Fixed headers

Some of them were probably generated some time ago and were not
up to date
CS2 backbone: small error in the error checking fixed
Merge branch 'master' of github.com:plumed/plumed2
Allow negative stride in atom ranges

One can e.g. specify ATOMS=10-1:-3

Closes #45
Regtest for ranges
Merge branch 'master' into walkers-mpi
removed a number of warning about int/unsigned comparison
CH3Shifts: methyl groups chemical shifts
CS2Back and CH3Shifts manual update
updated bibliography
CH3Shifts: enabled neighbor list
updated changes
COMMITTOR in the manual