Tile server: ensure timezone-naive time selector

[pont-us]

Fixes #555.

In tile._get_var_2d_array (used by new_color_mapped_image),
check for the possibility that the array uses datetime64 and the
indexer is timezone-aware. In this case, convert to a timezone-naive
indexer before performing the selection.

[Description of PR]

Checklist:

• [ ] Add unit tests and/or doctests in docstrings (not needed)
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions N/A
• [ ] New/modified features documented in docs/source/* N/A
• [ ] Changes documented in CHANGES.md N/A
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

Remember to close associated issues after merge!

[AliceBalfanz]

Tested locally and works well :) Thanks for fixing!

[forman]

• The natural home of this fix should be xcube.core.tile.parse_non_spatial_labels()
• I assume, the nature of the problem requires a more general solution. Hence, we need an understanding of the root cause and a general function that fixes the issue before timstamps are passed into xarray.Dataset/DataArray.sel().

[TonioF]

I really would like a test for this. Also, I'd expect this issue to arise in more places than this. Did you check the other places?

[pont-us]

I really would like a test for this.

OK, added.

[pont-us]

Did you check the other places?

There seem to be fewer of them than I feared. I found time selection used in three other places in the xcube codebase:

• core.select.select_temporal_subset
• core.timeseries.get_time_series
• cli.tile.tile

The last of these will be made safe by Norman's suggestion of moving the check to core.tile.parse_non_spatial_labels(), but the other two will require further work.

[pont-us]

The natural home of this fix should be xcube.core.tile.parse_non_spatial_labels()

OK, moved.

[forman]

but the other two will require further work.

Then let's just fix the server and enhance the issue #555 by your findings or create a new one.