Add epistemic uncertainty branches to NGA-Sub

Conditioned on completion of #85 (closed), update KCBG_20 and PSHAB_20 to return epistemic branches from calc()

changed milestone to %Coding 'til Christmas

added GMM label

added In Progress label

mentioned in merge request !216 (merged)

mentioned in merge request !225 (merged)

removed milestone %Coding 'til Christmas

added 2023 label

assigned to @jaltekruse

mentioned in issue #133 (closed)

changed milestone to %NSHM Hazard Tool launch

removed In Progress label

Resolved with !216 (merged) and !225 (merged) and othr recent work

closed