ENH: Tolerance threshold for masked array aggregation

Ability added to pass missing data threshold to the aggregation class.
Minor fixes based on review and extended usage
Second iteration of changes
Deprecated add_history
Merge pull request #820 from esc24/dep_history

Deprecate add_history()
Merge pull request #821 from cpelley/collapse_missdata_tol

ENH: Tolerance threshold for masked array aggregation
Fixed failing Aggregator tests on master.
Merge pull request #823 from ajdawson/fix-aggregator-tests

Fixed failing Aggregator tests on master.
Basic TimeConstraint(hour=...) capability