Improvements to NshmModel class

[chrisdicaprio]

• gmcm_nshm_v1.0.0.json and gmcm_nshm_v1.0.4.json are identical, keep one
• #95
• #97
• #89
• Consider: make NhsmModel logic tress and config optional --> psha_adapter would have to handle missing components of the model
• Consider: serialize/deserialize HazardConfig so that it can be defined by a file like the logic trees are for "official" model versions