Yambo spectra

[Jajar26]

In .../nomad/datamodel/results.py:

In the Spectra class, we have added 'Polarizability' and 'Dielectric function' to manage the different Yambo spectra types, as follows:

type = Quantity(
type=MEnum(
'EELS',
'XAS',
'XANES',
'EXAFS',
'XES',
'XPS',
'RXIS',
'Polarizability',
'Dielectric function',
config.services.unavailable_value,
),