rest api enhancements for composite resource type

[zhiyuli]

@sblack-usu @martinseul
I list feature requirements from apps' perspective. Feel free to split and/or move them to hydroshare repo if needed. Also, this might be a running list. Thanks

1. retrieve all metadata for a given aggregation: currently FilesSubEndpoint.metadata(self, file_path, params=None) only returns core metadata without file-type specific metadata

2. filtering based on aggregation metadata: return a list of all aggregations from all accessible composite resources that meet certain criteria, such as return all geographic feature aggregations whose Geometry Type is POLYGON and Feature Count ==1

3. download all files for an aggregation: download a specific geographic feature aggregation from a composite resource

[zhiyuli]

@danames @kjlippold
please review and add your comments/requests when you get a chance

[kjlippold]

Here's a few other things I'd like to see, if there isn't already some way of doing them.

1. Include a list of aggregation types present in the resource in a get resource request, like what appears under "content types" on the resource landing page.

2. This is related to Drew's second point, but the ability to get a list of all resources containing specified aggregation types. (Different than getting a list of aggregations).

3. Ability to list all aggregations in a given resource.

4. Ability to create, edit, and delete aggregations in a given resource, beyond just automatically creating them when files are uploaded.

5. There should probably be a way to get a list of aggregation types, similar to how there's a way to get a list of resource types.

[kjlippold]

I think a lot of these features need to be added to HydroShare's rest api before they can be added to the rest client.

[sblack-usu]

ok, I'm seeing basically two categories to these requests:
Discovery
Resource Actions

Most of the work here needs to be done on the rest api in hydroshare. I'll work on the Resoruce actions withing hydroshare/hydroshare#2575. Once I wrap that up, I'll create an issue for discover. I'd like to setup a rest api endpoint that just routes to @alvacouch's discover, where most of these requests are already implemented or should be implemented there.