[SIMBAD] Add filter names as possible votable fields

[ManonMarchand]

I'd like to tweak a bit the way I handled the fluxes votable fields deprecations in the SIMBAD rework.

In the current version, this:

from astroquery.simbad import Simbad
simbad = Simbad()
simbad.add_votable_fields("flux(u)")

works with a warning that does not say how this should be replaced.

This PR adds new votable fields for the SIMBAD's filters. And thus we can provide a clearer warning that points to this new replacement. This uses the allfluxes table from SIMBAD's relational schema (http://simbad.cds.unistra.fr/simbad/tap/tapsearch.html) that only contains the magnitude value.

There is also a clearer error message for the deprecated flux_**() votable fields that were falling in the generic error message for unknown fields. There is now a message that says that the replacement is the votable field flux which adds the whole flux table with every information covered by these former votable fields.

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In addition, the allfluxes votable field was broken

from astroquery.simbad import Simbad
simbad = Simbad()
simbad.add_votable_fields("allfluxes")

would make every query fail because the names of the columns are casefolded while this table is an exception in the database and is case-sensitive. This is fixed in this PR.

[keflavich]

This change looks great to me. Thanks for your detailed attention to the user experience! It's especially good to see users no longer being asked to write SQL.

[bsipocz]

Looks all good pending a minor clarification. I don't think that changelog is required as the VO backend refactoring hasn't yet been released yet.

[bsipocz]

maybe rephrase the description to make sure it's clear that filter is for an optical filter rather than a VO/any other computing filtering expression. I expected the latter :)

(maybe if possible, then rename it to filter name? that would make it unambiguous)

[ManonMarchand]

Ah sure! Did not think about that ^^
This is renamed (and I also tried to make it a bit clearer in the documentation)

[bsipocz]

Thanks!