added gap size

[shimwell]

Proposed changes

This adds the ability to add a gap between the TF and the divertor.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [ ] Pep8 applied
• [ ] Unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

[shimwell]

@katie-taylor @billingsley-john I think this is ready for review

[billingsley-john]

Looks like a very useful addition!

[codecov[bot]]

Codecov Report

Merging #845 (b53c7fe) into develop (e459768) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #845   +/-   ##
========================================
  Coverage    94.68%   94.68%           
========================================
  Files           75       75           
  Lines         5401     5402    +1     
========================================
+ Hits          5114     5115    +1     
  Misses         287      287           

Impacted Files	Coverage Δ
paramak/parametric_reactors/ball_reactor.py	98.91% <100.00%> (+<0.01%)	:arrow_up:

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