Fix 241 (#248)

* speed up of sharing and applying in plumed core
the main differences are:
- new member of class atoms to track unique local atoms (i.e. all the atoms needed by plumed action in a domain)
- positions are retrieved only for unique local atoms
- forces are zeroed only for unique local aotms
- forces are applied only for unique local atoms
in a system of 200K atoms on 4 mpi rank with plumed using only 4 atoms the
sharing and applyign time improves by 2 order of magnitudes

* removed leftover printf

* core: make it a bit faster also without domain decomposition

* astyle

* fix-241: unique atoms are update only if:
- particle decomposition has been updated
- domain decomposition has been updated
- replica-exchange step
- neighbour lists have been updated
overall this should make the sharing code even faster

* actionAtomistic have now a list of local atoms that is updated on:
- neighbour list step
- update of the pd/dd list
this allow to simplify the loop for the generation of global local list of unique atoms

* atoms: small tweaks

* fix: reallocate memory less often

* fix: for the scalar case there is no need to generate the list of unique atoms

* style

* few comments