Added atom_sites_* structure id key

[rowlesmr]

Added linked key *.structure_id data names to ATOM_SITES_AXES, ATOM_SITES_MODULATION, and ATOM_SITES_ORTHO.

Did not add to ATOM_SITES_DISPLACE_FOURIER, as it is a deprecated category - see #50
Did not add to ATOM_SITES_ROT_FOURIER, as it is a deprecated category
Can be added if needed.

[rowlesmr]

ATOM_SITES_ROT_FOURIER has no keys at all, and consists of only one deprecated data name. Category should be deprecated. cf #50

[jamesrhester]

Same comment as on #45 : should we add a short phrase to the *.structure_id data names to state that they can be omitted?

[rowlesmr]

Same comment as on #45 : should we add a short phrase to the *.structure_id data names to state that they can be omitted?

already done for ATOM_SITES_AXES, ATOM_SITES_MODULATION, and ATOM_SITES_ORTHO

[rowlesmr]

Some changes got smushed somehow. Put them back in.

[rowlesmr]

Did not add to ATOM_SITES_DISPLACE_FOURIER, as it is a deprecated category - see #50 Can be added if needed.
Did not add to ATOM_SITES_ROT_FOURIER, as it is a deprecated category

Should it be added here for backwards compatibility?

[jamesrhester]

Should it be added here for backwards compatibility?

No. No legacy software knows about _structure.id.