Dictionary keys for probes (euclidlib internal format)

[gcanasherrera]

We are aiming to use python dictionaries with tuples as keys (from now on, colloquially referred as the internal euclidlib mapping). Example: the key for the angular power spectrum for galaxy clustering in the first redshift bins would be:

('P', 'P', 1, 1)

We need to agree on:

1. If possible, keep consistency among probes (for instance, keep 4 keys for all photometric probes, including correlation functions and mixing matrices)
2. If possible, keep the same 'keys', as we already separate probes in the API (example: el.photo.angular_power_spectra)

On top, we need to agree on possible keys for:

• Photometric correlation functions
• Spectro power spectra
• Spectro correlation functions

UPDATE:

• 'POS': angular clustering positions instead of P
• 'SHE': cosmic shear (alternatives are 'SHE_E', 'SHE_B', 'SHE_plus', 'SHE_minus')