Don't guess instantaneous time coordinate bounds

[Scott Wales (ScottWales)]

Guessing time bounds for instantaneous fields causes problems for METplus and Verpy when working with CSET-processed files.

Fixes #2101

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[Scott Wales (ScottWales)]

There are some failing tests with this change:

• tests/operators/test_collapse.py::test_collapse_by_validity_time_no_common_points, tests/operators/test_misc.py::test_difference_no_time_coord, tests/operators/test_imageprocessing.py::test_SSIM_no_time_coord
  Tests are extracting cubes using times not present in the file - file has [2022-09-21 03:00:00, 2022-09-21 04:00:00, 2022-09-21 05:00:00], tests are trying to load times on the half hour. An error in the test code? Would be simple to change the loads to be on the hour instead.
• tests/operators/test_plot.py::test_get_start_end_strings_bounds
  Expected change, bounds are no longer being created for the instantaneous time. The function could be updated to guess the time bounds to retain the previous behaviour, or the test values updated to reflect that these bounds are from an instantaneous field.

James Frost (@jfrost-mo), JorgeBornemann (@JorgeBornemann) any preferences on how these are resolved?

[JorgeBornemann (JorgeBornemann)]

* **tests/operators/test_collapse.py::test_collapse_by_validity_time_no_common_points**, **tests/operators/test_misc.py::test_difference_no_time_coord**, **tests/operators/test_imageprocessing.py::test_SSIM_no_time_coord**
  Tests are extracting cubes using times not present in the file - file has `[2022-09-21 03:00:00, 2022-09-21 04:00:00, 2022-09-21 05:00:00]`, tests are trying to load times on the half hour. An error in the test code? Would be simple to change the loads to be on the hour instead.

Maybe duplicating the test? one that loads cubes that correctly have bounds in time and reads whatever times are required and another test that loads cubes that don't have bounds and reads times on the instant times?

* **tests/operators/test_plot.py::test_get_start_end_strings_bounds**
  Expected change, bounds are no longer being created for the instantaneous time. The function could be updated to guess the time bounds to retain the previous behaviour, or the test values updated to reflect that these bounds are from an instantaneous field> 

I would prefer to update the test values, but we may need to retain the coverage of the initial test, looking into cubes that require bounds, so perhaps also duplicating the test?

[Scott Wales (ScottWales)]

Thanks JorgeBornemann (@JorgeBornemann). I have fixed up the first set by correcting the times in the test and adding a new test in test_reads that makes sure fields are loaded when the time constraint is inside the time bounds.

For the plotting tests there's adjacent tests for fields with and without bounds, so I've moved things around so that each of these is getting the correct cubes.

[JorgeBornemann (JorgeBornemann)]

The changes are sound. For me they are good to implement but, as they touch a very generic operator I leave the approval to James Frost (@jfrost-mo) who is more familiar whit it.