Fixing get_atomID method.
Fixing get_atomID method.

Changing the method signature to use 1 positional argument
for the `atomID` attribute. Fixed code so the atomID index
is looked in the `atom_ids` list and used to return the
`Atom` from the `Atoms` list.

scikit-nano/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano