Adding aliases for Observables

[SBouchard01]

This PR adds aliases for the Observables classes, as described in #116

Warning

This still needs some works and requires #119 and #118 to be merged in cosmodesi, as this branch is a child of cosmodesi-bgs_obs (which is a child of cosmodesi-emc_obs)

If we want to avoid confusion, we need to clarify some naming conventions :

1. The alias (or stat_name) of each observable needs to be unique (it's not the case in the VERSUS observables, this is why they don't have aliases yet - @ntbfin00 )
2. The name of a file containing the Observable class should be different from the alias value, to not confuse the namespace (see Observables aliases #116 discussion on this)
   In some cases, it makes sense to rename the observables aliases and/or stat_name values (e.g. pk could be a bit more clear than spectrum for the power spectrum observable ? 🤔);
   In other cases, we could just rename the file name (e.g. minkowski.py could become minkowski_functionals.py)
   This includes: tpcf, bispectrum, dd_knn, minkowski, projected_tpcf, recon_spectrum, spectrum, wst

Important

This would require to recreate the compressed files !

What needs to be done to have this draft ready for review

1. Each observable needs to have an alias (that ideally should match the stat_name value of its observable but this is not mandatory, just less confusing)
2. The aliases and file names should be different

[SBouchard01]

Warning

VIDE aliases will need to be implemented after merging #146, I left the current classes names for now

Also, I commented out the GalaxyOverdensityPDF as there is no pdf.py file in acm.observables.emc

As mentioned in this comment, the namespace can now be a bit confusing for the files that also match the stat_name/alias of the Observables. From what I found on those implementations it's still OK to do it, but we just need to be careful about that