Explicit check of input file update times when creating weather symbols

[bayliffe]

This PR includes changes that allow the weather symbols CLI to check that at least one of the input files is more recent than an optionally provided update_time. If no files are more recent than this, the CLI returns without output, preventing the recreation of a weather symbol that includes no more recent input data.

This implementation needs the addition of tests if we were to adopt it.

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0419260) 98.38% compared to head (d1358d6) 98.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.6.x #1964 +/- ## ======================================= Coverage 98.38% 98.38% ======================================= Files 123 123 Lines 11798 11798 ======================================= Hits 11608 11608 Misses 190 190 ```

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[bayliffe]

Not necessary.