Add. `date_start` and `date_end` properties to metric classes.  Utilize this in metrics_manager module rather than handling timestamps
Fix. Order of args to get_agg_key()
Mod. Move tool method build_key_tree() into engine.py.
Fix. If the retrieval of the cohort id "last updated" timestamp  raises an exception pass without the value.
Fix. Removed old print statements for emitting output.
Fix. Representation of 'aggregator' to be independent of metric.