WIP: refactor engine api with flask-restful

flask-restful provides a clear way of expressing endpoints as resources supporting multiple http methods.

* update to use flask-restful
* early start to parsing messages (note: compact json-ld requires pyld ad rdfli_jsonld)
* draft example json-ld graph for submitting job request

todo:

- [x] finish converting `get_file` and `get_info` to flask-restful
- [ ] define response format as json-ld graph - jobs 
- [ ] do different workflows have different response formats (i.e., more than job id?)
- [ ] how do given workflows define/register their "input graph" (plugin framework?)
- [ ] add tests...
- [ ] ...