Create distance-dependent GMMs for CONUS 2014 stable crust

With the conversion to GMMs returning a logic tree of ground motions, we can support the distance-dependent GMM logic trees used for stable crust in the 2014 CONUS NSHM. See nshmp-haz#563

• Create single GMMs for fault and grid source types that wrap the individual GMMs required and internally manage distance-dependent weighting.
• Expose mean and sigma of the individual GMMs to avoid singleton logic tree boxing and unwrapping.

added GMM label

changed the description

changed the description

Exposing mean and sigma canceled; see #264 (closed).

marked the checklist item Create single GMMs for fault and grid source types that wrap the individual GMMs required and internally manage distance-dependent weighting. as completed

mentioned in merge request !434 (merged)

mentioned in commit d0fbc67e

closed with merge request !434 (merged)