Migrate gmm package

Created by: pmpowers-usgs

This is a large package and should have few if any dependencies outside the package.

• Review unit test coverage before migration
• Once moved, refactor MultiScalarGroundMotion to ScalarGroundMotion; the default implementation will support arrays of mu and sigma; a singleton implementation will be used to support single mu/sigma GMMs

changed milestone to %nshmp-lib Review Ready - Sprint 2

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changed milestone to %NSHM App Update

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This is vague. New issue created for ScalarGroundMotion; see #85 (closed)

closed

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