Fix.  Add logging library.
Fix.  Minor indentation issues.
Fix. If any key-word argument is empty, set it to default.
Add. Store a class variable that contains a default time range for metric objects `DEFAULT_DATA_RANGE`
Add. logger + docs.
Mod. moved `date_start` & `date_end` parameter definitions into user metric super class.  These will apply to all metrics.
Fix. Remove reference to setting datetime defaults outside of UserMetric class.