Improvements for xarray provider

[barbuz]

Overview

Using the xarray provider to manage Zarr data extensively, I have noticed and fixed a few bugs and added a couple of features that could be useful, so I would like to contribute these changes.

Additional information

Summary of changes:

• Avoid crashes when the time dimension is not cf-compliant by trying to reload the data with decode_times=False
• Allow managing data without a time dimension at all
• The provider was already automatically managing subsets on coordinates sorted in reverse order, now it can do so also when applying a bounding box
• Fixed bug where some values where not converted to float64 before generating json output
• Used a named temporary file to generate netcdf output to allow use of netcdf4 engine
• A couple of minor changes
• Allow requesting for netcdf or zarr output format regardless of the data format

This last change is the trickiest. I had to patch the APIRequest class to advertise the capability of producing more than one kind of output format for coverage data. At the moment I have hardcoded the two formats supported by the xarray provider, but eventually it would be nice to allow each provider to advertise an arbitrary list of output formats; I'm proposing this change as an initial implementation and a future PR could be able to expand on it and make this available to all providers. Please let me know what you think about this.

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[tomkralidis]

@barbuz are you able to rebase this PR? I'd rather cover the format handling as part of another issue/discussion. Are you able to reduce this PR to the pure Xarray fixes?

[barbuz]

@barbuz are you able to rebase this PR? I'd rather cover the format handling as part of another issue/discussion. Are you able to reduce this PR to the pure Xarray fixes?

I have rebased this reverting the changes to the format handling. I have opened a separate issue to discuss format handling for coverage providers in general: #1821