xcube WCS

[thomasstorm]

• (X) Replace usage of xcube.core.gen2 by Common tile computation #739. This will fix many of the issues listed below as this is a threadsave and scalable approach to fetching data.
• Add tiles API requirement
• Adapt to xcube coding style: always pass kwargs with keyword rather than position
• Include a dataset's bounding box in DescribeCoverage operation.
• Recognize WIDTH, HEIGHT and RESX, RESY (opt. DEPTH and RESZ) that are mandatory if INTERPOLATION is not "none". Currently, only BBOX is used, sell also displacement in screenshot below.
• Clarify meaning of gml:CodeListType for formats in capabilities XSD.
• Get rid of class xcube.webapi.ows.wcs.controllers.CoverageRequest (as it is no more than a wrapper of the query)
• Fix unit tests (mostly broken due to changes in reported capabilities).

Work items done:

• Fix extra WCS service metadata entries in server config, e.g., move into config.py with defaults as code constants
• Configuration of WCS keywordsmetadata shall not be mandatory
• Auto-close temp files in context manager
• Raise ApiError.BadRequest if request validation fails
• Refactor operation GET /wcs/kvp - is is too long and barely maintainable this way
• Adapt to xcube coding style: always import only single item from dep or own package
• Also variable titles should include the dataset name, otherwise they are indistinguishable in , e.g., QGIS
• Include supportedInterpolations in DescribeCoverage operation.
• Make TIME optional as not all datasets have a time dimension.

Work item potentially obsolete after (X) has been done.:

• The way the dataset paths are parsed from datasets_ctx.get_dataset_configs() is correct only for edge cases (namely, only for filesystem data stores). We must generalize this, i.e., allow for reusing the dataset configurations from xcube server directly in xcube cube generator requests of xcube.core.gen2.
• Remove tiles API requirement
• Fix tornado.iostream.StreamClosedError: Stream is closed issue (likely reason: temporary file removed before Tornado finishes request)
• Opt: allow using in-memory file I/O to avoid temporary files entirely as they likely produce problems
• Fix root cause of datasets' history global attribute not being JSON-serializable (set by xcube Generator)
• Fix KeyError: Timestamp('2017-01-25 09:35:51') raised by pandas
• Fix TypeError: unsupported format string passed to NoneType.__format__ after cube generation (message=f'Cube generated successfully')
• Fix xcube.core.store.error.DataStoreError: Failed to open dataset './demo.conc_chl.zarr': group not found at path ''

Typical QGIS request:

GET /wcs/kvp? 
  service=WCS
  &version=1.0.0
  &request=GetCoverage
  &SERVICE=WCS
  &VERSION=1.0.0
  &REQUEST=GetCoverage
  &FORMAT=geotiff
  &COVERAGE=local.conc_chl
  &TIME=2017-0116T10:09:21Z
  &BBOX=-180,-90,180,90
  &CRS=EPSG:4326
  &RESPONSE_CRS=EPSG:4326
  &WIDTH=920
  &HEIGHT=487

Checklist:

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