add/mod. 'restrict' key to _param_types / pep8.
rm. existing restrict parameter modification for threshold and survival.
rm. start and end time params from method.
mod. synced _param_types definitions with UserMetric object attributes
add. 't' to default metric attributes.  This is the hours over which to measure any metric since an event (typically registration).
fix. ThresholdArgsClass constructor.
rm. _data_source_ attriubute. deprecated.