Support evaluating with incomplete gold standards

[jnothman]

A user on the TAC mailing list wished to evaluate a mix of 2014 and pre-2014 EL tasks, such that precision should indicate precision of linking/clustering, ignoring spurious mentions.

While it is not hard to remove spurious mentions using grep, this may be worth facilitating in one of the following ways:

1. a command to output the subset of a dataset that aligns to the gold standard.
2. an --ignore-spurious flag to evaluate, significance and confidence.
3. an is_aligned attribute on Annotations that is True for all gold annotations and set with respect to some gold standard when loading annotations from a system output.

(3.) would appear to be most flexible and in line with current design.