modulate verbosity of astroquery.mast.Observations.download_products()

[afaisst]

When using download_products() from the astroquery.mast module there is currently no way of modulating the printed output. It currently prints for each downloaded product "Downloading URL ..." (or that file already exists and is not being downloaded). This output can be annoying long if 100s of data products are being downloaded.

It would be good to have an argument in that function so that the user can opt out of this very lengthy output.

[bsipocz]

cc @astrojimig @snbianco