Make sure all documentation and functions consistent re: 0 or 1-indexing

[ajratner]

In the LabelModel, we have 0 reserved as special value for abstains; then in subsequent EndModel calls, we use 1 indexing to be internally consistent. However we should do a cleaning pass to make sure that (a) this is consistent in documentation / checks / etc, and (b) doesn't trip up people just using the EndModel with non-probabilistic labels, who might expect 0-indexing.

[henryre]

Active development is being moved to our main repo, https://github.com/snorkel-team/snorkel, where we standardized -1 as abstain so that the label model and downstream classifiers can use the same non-abstain class indices.