NEB upgrade

Hi all - I added an option to the NEB command to read initial configs
from multiple input files (w/out interpolation) or from no files
at all (making it easy for each replica to read its config
from a data, dump, or restart file).

The neb command syntax changed a bit to enable this.

Also the file format for atom coords has an added line,
to make it more compatible with other similar LAMMPS input files.

Also, the error checking on valid input coord files is now better.

There were all options people were asking for.

So if NEB-in-LAMMPS users can try this out, feedback would
be appreciated.

The ability to have multiple procs/replica is still to come.

Also, a couple Qs for NEB folks:

a) what is the typical optimal choice for a NEB timestep when
running damped dynamics to minimize iterations? I’m thinking it is larger than
a normal timestep for dynamics

b) has anyone used NEB successfully with load forces on the
replicas, e.g. fix addforce? It’s not clear to me that this will
work with a damped dynamic minimizer

c) is it useful in the NEB context to allow the box shape/size
to change between the initial/final replica states? This is currently
not allowed, again for damped dynamic minmizers, but is something

we could think about …