make sure slice on osgb is ints

[peterdudfield]

Pull Request

Description

fix for forecasting error

Helps with openclimatefix-archives/nowcasting_forecast#207

How Has This Been Tested?

CI tests, but not sure it will solve the bug

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[codecov]

Codecov Report

Merging #728 (2b0490d) into main (db2af0c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #728   +/-   ##
=======================================
  Coverage   92.86%   92.86%           
=======================================
  Files          49       49           
  Lines        3488     3488           
=======================================
  Hits         3239     3239           
  Misses        249      249           

Files Changed	Coverage Δ
nowcasting_dataset/data_sources/data_source.py	89.63% <ø> (ø)

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