Squashed 'node_mapper/nomadic_recording_lib/' changes from bd68f2d..d23c57d

d23c57d added sorting and ZeroCenteredIndex to ChildGroups
734df7a added a dict-like object that calculates indecies of its items centered around 0.  'ZeroCenteredGroup' seemed like a decent name
39adbea subtree remote fetch raising exceptions. temporarily removed

git-subtree-dir: node_mapper/nomadic_recording_lib
git-subtree-split: d23c57d677480f6144afc8475594a1b5ad189256
Merge commit '435cc9e566d8fd834996cbaea7baf0c2ad4baa03'