EPIC: graphql API exposes core features of the nzshm-model library

[chrisbc]

We want to share the info contained in nzshm-model for non devs, so this API should provide this with:

Done when:

• graphql API exposes source_logic_tree features of the nzshm-model #22
• list available model versions see Feature/api based on nzshm model #20
• get current model version
• feature/gmm_logic_tree classes #24

next

• use nzshm-model registry to get hash_digests for gmm_branch ids (as gsim_name +gsim_args is very long) #26
• list source tags (for UI filters) #27
• list gmm names #28

future ideas

• get openquake config files
• resolver for hash_digest values
• create new model by filtering from existing (for THP users?)
• create new model by specifying LTB hash_digests
• tectonic regions