Implement STAC and OGC API compatibility improvements

[pont-us]

Closes #904 .

• Expand the STAC catalogue coverage links to provide links for multiple formats.
• For TIFF and PNG OGC coverages, apply the squeeze method to remove any singleton dimensions.
• Add support for the scale-factor parameter to the main coverage endpoint.
• Improve handling of bbox parameters: they are now applied to the axes in the order that they appear in the dataset, as required in the specification. Inverted y-axes are also handled.
• Handle half-open datetime intervals.
• More parameter checking: various invalid parameters will now result in a 400 response rather than a server-side error.
• Produce a 501 response for parameters which are not yet implemented.
• Add media types to coverage links.
• Return a 400 Bad Request response for attempts at subsetting on non-existent axes.
• Handle quotation marks in coverages subset parameter, if present.
• Use crs_wkt when determining CRS, if present and needed.
• Give a 400 Bad Request response on requests for non-existent properties (i.e. variables).
• Change default subsetting and bbox CRS from EPSG:4326 to OGC:CRS84.
• Implement reprojection for bbox.
• Run datetime parameter values through ensure_time_index_compatible function to fix timezone-aware / -naive incompatibilities
• If writing an image from a Dataset with a single variable, write the relevant DataArray rather than the whole Dataset (avoids errors due to additional "bnds" dimensions).
• Return the appropriate HTTP responses for requests which result in no data or too much data.
• Add crs and crs_storage properties to STAC data.
• Reimplement subsetting to conform more closely to the standard and to separate request parsing more cleanly from data operations.
• Set the Content-Bbox and Content-Crs headers when returning a coverage.
• Support safe CURIE syntax for CRS specification.

Checklist:

• [x] Add unit tests and/or doctests in docstrings
• [x] Add docstrings and API docs for any new/modified user-facing classes and functions
• [x] New/modified features documented in docs/source/*
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (add6f00) 93.32% compared to head (851859f) 93.40%.

:exclamation: Current head 851859f differs from pull request most recent head e2c4009. Consider uploading reports for the commit e2c4009 to get more accurate results

Files	Patch %	Lines
xcube/webapi/ows/coverages/controllers.py	96.73%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #905 +/- ## ========================================== + Coverage 93.32% 93.40% +0.07% ========================================== Files 447 449 +2 Lines 34924 35335 +411 ========================================== + Hits 32594 33003 +409 - Misses 2330 2332 +2 ```

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