corrected the calculation of the minimum coverage observed
Add 'condition_pairs' to DifferentialExpression

Instead of using condition_labels_string and condition_model_ids_string
as inputs to DifferentialExpression models, start using an is_many
condition_pairs input. Read 'doc' string of the inputs for more details.
This commit keeps both the old and new inputs to aid backfilling.
Update ClinSeq interface to DifferentialExpression
Remove old condition strings from DiffExp
Update define docs test for DiffExp
Updated class browser cache.
Updated `genome` tab completion.