Update reporting of IMTs for a model

This line generates the IMT parameter list that is returned by hazard and disagg usage metadata: https://code.usgs.gov/ghsc/nshmp/nshmp-haz/-/blob/main/src/main/java/gov/usgs/earthquake/nshmp/www/source/SourceService.java?ref_type=heads#L81

It actaully (incorrectly) returns the complete set of IMTs supported by all GMMs in the model, not the intersection across all GMMs. We've gotten lucky in that all models to date are limited to MPRS IMTs. This list is used to populate the disagg app IMT selector.

This line is used to set the list of IMTs when calculating hazard if a user does not provide any IMTs to the hazard web service: https://code.usgs.gov/ghsc/nshmp/nshmp-haz/-/blob/main/src/main/java/gov/usgs/earthquake/nshmp/www/hazard/HazardService.java?ref_type=heads#L219 This is predicated on having a calc-config in a model with explicit IMTs listed; if absent the config default MPRS is used.

Both metadata and default calculation (no IMT specified) should use the same model-derived set of IMTs.