Extra user-agents in BaseVOQuery

[ManonMarchand]

Hi astroquery maintainers!

We'd need to allow some users (ex: a whole software that depends on astroquery) to add their own user-agent when they use the SIMBAD module. I think this implies a modification to the BaseVOQuery class. Would you be ok with such a change?

[keflavich]

Yes, that's fine - if there's a need for specific agents, we should be able to accommodate that.

[bsipocz]

Sure, especially if it's an addition, so we would still ship the astroquery and pyvo info.

(But for simbad you are the boss and can say what info you want to see from someone using astroquery as their client)