Update ground motions to support epistemic uncertainty

• Replace current: ScalarGroundMotion --> DefaultScalarGroundMotion --> MultiScalarGroundMotion with GroundMotion and factory class GroundMotions
• GroundMotions class to include methods for collapsing GroundMotion logic trees
• Refactor GMMs to return LogicTree<GroundMotion>; many models are singletons
• Build USGS uncertainty model into NGA-West2 with custom *_USGS flavors; *_BASIN models will implicitly include epistemic uncertainty model; remove UncertaintyModel
• Remove epistemic-model and epistemic-tree from gmm-config
• Remove gmmUncertainty flag from calc config
• Convert other GMMs that return MultiScalarGroundMotion (e.g. NGA-East) to build GroundMotion logic tree
• Propagate changes to ground motion classes into calculations and transforms
• Remove GmmProcessor