Add zSed to site data

• Add zSed to GmmInput; like z*p* values, NaN is default
• Add zSed handler to SiteData

See ghsc/nshmp/nshms/nshm-conus#12 (closed)

@olboyd

added GMM label

changed milestone to %Coding 'til Christmas

mentioned in issue #132 (closed)

removed milestone %Coding 'til Christmas

added 2023 label

changed milestone to %NSHM Hazard Tool Launch - 2

mentioned in merge request !253 (merged)

marked the checklist item Add zSed to GmmInput; like z*p* values, NaN is default as completed

marked the checklist item Add zSed handler to SiteData as completed

mentioned in commit 281d188f

closed with merge request !253 (merged)