Add SPHERE form to ESO query

[keflavich]

The SPHERE instrument specific form has just been created and it needs to be added.
http://archive.eso.org/cms/eso-data/instrument-specific-query-forms.html

[jwoillez]

Now that SPHERE is in the web page mentioned above, I don't think there is any extra thing we have to do.

>>> from astroquery.eso import Eso
>>> eso=Eso()
>>> eso.list_instruments()
['fors1', 'fors2', 'sphere', 'vimos', 'omegacam', 'hawki', 'isaac', 'naco', 'visir', 'vircam', 'apex', 'uves', 'giraffe', 'xshooter', 'muse', 'crires', 'kmos', 'sinfoni', 'amber', 'midi', 'harps']

Close issue?

[bsipocz]

Yes, it was added to the instrument list in #551 (as it caused remote test failing). I haven't noticed this open issue then.