Ensure the GUI's open dialog box always shows a human readable date, even when using MJD values from a FITS header

[ortk95]

When opening a FITS file with the GUI's open dialog, the target, date and observer fields are auto-populated (if possible) using information from the FITS header. Previously, this meant that if the header contained dates in MJD format, the date field would show the date as a numeric value. This worked completely fine, but made it difficult to double check or understand the auto-populated value.

Therefore, this update will automatically convert auto-populated MJD values (e.g. 61043.93858073495 to a human readable date (e.g. 2026-01-03T22:31:33.375500), making it much easier for the user to understand what the value means.

This has no functional effect, but just makes things easier to use.

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This is implemented by re-using the same code that is used to convert a MJD to a SPICE-compatible date. The date standardisation code has been extracted from BodyBase.__init__ and now lives in the classmethod BodyBase._standardise_utc_to_string, where it can easily be re-used by other parts of the code as necesary. Extra unit tests have been added to ensure this conversion works as expected across different inputs, and provides a stable output.

Closes #585

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Pull request checklist

• Add a clear description of the change
• Add any new tests needed
• Run spell check on new text visible to user (documentation, GUI etc.)
• Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Coverage Report for CI Build 29920886187

Coverage remained the same at 99.61%

Details

• Coverage remained the same as the base build.
• Patch coverage: 13 of 13 lines across 1 file are fully covered (100%).
• No coverage regressions found.

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Coverage Stats

Relevant Lines:	3470
Covered Lines:	3458
Line Coverage:	99.65%
Relevant Branches:	890
Covered Branches:	885
Branch Coverage:	99.44%
Branches in Coverage %:	Yes
Coverage Strength:	14.88 hits per line

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