Add support for molecule templates

[mphoward]

PR #97 uses LAMMPS JSON molecule templates to demonstrate initialization of an atomistic simulation. It would be nice to natively support parsing of these templates in lammpsio from either the data-file format or the JSON format. This data should be read into an object that supports some kind of replication... this might be a simple shift of the atom IDs internally (plus ensuring a deep copy mechanism), or it could be a mechanism to replicate 1 or more molecule templates a certain number of times.