Xarrayedr covjson

[adanb13]

Overview

• fixes coverage json output for xarray edr
• changing of domain axis time -> t instead of time and providing a data array of time values instead of just start/stop/num
• replacing NaN vlaues with None so that they can properly be parsed by JSON as null

Related Issue / discussion

Coverage Json being returned from xarray edr provider is currently invalid

Additional information

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