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.