Common namespace for multiple VOs

[jjensenral]

(Regarding the assertion of group information, see also #10 and #25)
Following G002 (G069), a user who has multiple groups from the same attribute authority is likely to end up with

urn:example:foo:group:a
urn:example:foo:group:b
urn:example:foo:group:c
urn:example:foo:group:c:c1
urn:example:foo:group:c:c2

which, if/when all are asserted, is considered verbose (which may not matter when userinfo is used). Where today it could look like

{
  "groups": ["urn:example:foo:group:a","urn:example:foo:group:b","urn:example:foo:group:c","urn:example:foo:group:c:c1","urn:example:foo:group:c:c2"]
}

it could change to, say,

{
  "groups": {"urn:example:foo:group": ["a","b","c","c1","c2"]}
}