[pygeoapi] OGC API branches update

[Tom Kralidis]

Hi all: there are a few branches representing work done at OGC API Hackathons,
some of which were WIP pull requests.  I've since closed their relevant PRs and
kept them as dedicated branches until they are ready to merge.  Below is an
overview of each branch:

- coverages: implements OGC API - Coverages using rasterio and xarray backends
- gen-catalog: implements OGC API - Records by generating catalogue of
resources resident in config
- manager: implements OGC API - Processing manager functionality (for
async and job management)
- stac: basic STAC implementation

Work on each of these branches were in the context of a 2-3 day
sprint, so they are
incomplete.  Moving forward on any of these branches require:

- relevant HTML representation/Jinja2 templates
- tests
- documentation
- updating against current master
- review against the current state of the given OGC API specification

Hope this helps.  Help / contributions welcome.

Thanks

..Tom