Get rid of all XML support

[prioux]

We have some control APIs that use XML and the main CBRAIN API also supports returning information in XML. However, XML documents break when keys are purely numeric. So we had to resort to horrible things like creating string like "T#{id}" as keys in some structures just so they could be serialized as XML.

If we just get rid completely of the XML support and only support JSON, this problem goes away and we can simplify the code.

[prioux]

Tasks prerequisites are an example of that.