Slight tweaks to the typing and docstrings, to stop mypy complaining!

[JackKelly]

Pull Request

Description

How Has This Been Tested?

• [x] Yes, unittests pass

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] 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
• [x] I have checked my code and corrected any misspellings

[codecov-commenter]

Codecov Report

Merging #696 (74f9a20) into main (0e22b7b) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #696   +/-   ##
=======================================
  Coverage   93.24%   93.24%           
=======================================
  Files          48       48           
  Lines        3388     3391    +3     
=======================================
+ Hits         3159     3162    +3     
  Misses        229      229           

Impacted Files	Coverage Δ
nowcasting_dataset/data_sources/fake/batch.py	86.66% <100.00%> (+0.10%)	:arrow_up:
...owcasting_dataset/data_sources/fake/coordinates.py	90.62% <100.00%> (+0.30%)	:arrow_up:
nowcasting_dataset/geospatial.py	100.00% <100.00%> (ø)

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