Proposed patch to allow specifying the site keyword in vso_get

The site keyword allows to select from which institute you desire to get the data. For example specifying site='rob' allows to download SDO data from the Royal Observatory of Belgium.

Not specifying the keyword will download the data from the regular location.

The site keyword will be added to the Info part of the request to the VSO. This required a change in http://docs.virtualsolar.org/WSDL/VSOi_rpc_literal.wsdl that has already been done by Joe Hourcle.