Fix and update featureset tests

- Fix comments for various tests that were duplicated due to copy and paste
- Add new test to ensure labels are copied during merging when one featureset has no labels
Fixed typo.
Modify FeatureSet len()

- Modify len() to return the number of examples, not the number of features.
- Update test_length accordingly.
Fixed filtering and added tests

- Fixed filter() to actually keep the ids/classes specified instead of removing them.
- Added tests for filtering.