Changing `assume_unique_atoms` to `assert_unique_ids`.
Changing `assume_unique_atoms` to `assert_unique_ids`.

Continuing updates to DATAFormatSpec class to support the
full format spec of the LAMMPS data format.

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