Add fault section service endpoint for loaded model

We want the Source Model --> Data Mapping application to display the GeoJSON features associated with each model. This behavior replaces rendering different tags of nshm-fault-section features.

We already have a http://.../features endpoint. Like the trees endpoint. The metadata returned is the listing of source logic tree IDs by source type, and the expectation is that a user then requests a particular .../features/ID. When requesting a source-tree ID from the features endpoint, the response includes the GeoJSON features for each branch in the requested source-tree (arranged as a map of `branchName/Path : Feature). In principal this is fine, but for normal faults in the western U.S., a user gets all three dip variants and for complex logic trees such as New Madrid, Wasatch, and Cascadia, the user gets all restitched combinations for each source-tree branch. This is a lot of redundancy and will complicate rendering without a bunch of model specific filtering.

For this issue, add a .../features/{group} endpoint that returns a FeatureCollection of just the features as supplied in the original source model. The {group} variable is a String parameter that can be any of [ "all", "fault", "interface", "zone" ]. This will require some refactoring in nshmp-lib to hold on to references to the original features used when instantiating the model.

Requires nshmp-lib#184 (closed)