Replace list-of-list reds with RedundantGroups everywhere

[steven-murray]

We should replace all list-of-list redundant groups with RedundantGroups objects everywhere.

One question would be whether we should make the get_reds function return a RedundantGroups object.

[jsdillon]

Relatedly, I have a feature request, which is to allow an access pattern like reds[0:10] that works with lists but doesn't currently work with this object.