is_stable() needs to be made global

[cbworden]

utils.py get_extent() calls is_stable() to determine if an epicenter is in a stable or active region. It currently only has the US stable region, Need to add other stable regions. Also could make it look at the Origin object for information to (possibly) override the simple point-in-polygon approach.

[emthompson-usgs]

I wrote this method just as a placeholder. It probably should use the attributes returned by STREC (or regselect/subselect or whatever it is called now). Actually, I think the best thing would be to use the selected GMPEs directly in the get_extent method and not use is_stable. We could use the GMPE(s) to then solve for the distance at which a threshold PGV (I used 4 cm/s to get the equations in get_extent) is exceeded for the event's magnitude and a relatively slow Vs30 (maybe 200 m/s).