Rename `time_slices_path` setting for consistency

[alexdewar]

All the other settings names for input file paths in settings.toml end with *_file_path, so let's rename time_slices_path to time_slices_file_path for consistency.

[alexdewar]

I've also renamed time_slices.rs to time_slice.rs to be consistent with the naming of other files.

[codecov[bot]]

Codecov Report

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

Project coverage is 76.41%. Comparing base (bf623cc) to head (f8b5cc5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #119 +/- ## ======================================= Coverage 76.41% 76.41% ======================================= Files 8 8 Lines 195 195 ======================================= Hits 149 149 Misses 46 46 ```

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