fix neb bugfix from Emile Maras

NEB was not working fine when using multiple proc
per replica and the keywords last/efirst or last/efirst/middle

I have corrected this in the enclosed fix_neb.cpp

I also slightly modified the nudging for this free end so that
it would be applied only when the target energy is larger than
the energy. Anyway if the target energy is lower than the energy,
the replica should relax toward the target energy without adding
any nudging.

I also modified the documentation according to this change.