Distinguish oxygen deficiency from oxygen excess

[CJBartel]

This effects 1802 entries.

Currently, data['target']['oxygen_deficiency'] == True when oxygen is deficient, in excess, or both.

Excess example (item 19390):
data['target']['material_string'] = 'GdBa1-xSrxCo2O5+δ'

Deficiency example (item 19700):
data['target']['material_string'] = 'SrFe0.75Mo0.25O3-δ'

Excess/deficiency example (19040):
data['target']['material_string'] = 'Sr1-1.5xCexTiO3 ± δ'

Functionality request:
Instead of 'oxygen_deficiency' (bool) as key, make new key which indicates the direction of oxygen nonstoichiometry - e.g., 'oxygen_nonstoichiometry' which can be False, 'excess', 'deficient', 'both'

[OlgaGKononova]

I made output for oxygen_deficiency attribute to be the sign before the variable, i.e. '-', '+', '±', '∓', so you can chose according to your analysis.