added the lightning_flash_accumulation_in_vicinity cell method missing from imengluk

[maxwhitemet]

Addresses #802

Testing:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (84a8944) to head (e595640).
Report is 49 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2064    +/-   ##
========================================
  Coverage   98.39%   98.39%            
========================================
  Files         124      133     +9     
  Lines       12212    13124   +912     
========================================
+ Hits        12016    12914   +898     
- Misses        196      210    +14     

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[lambert-p]

I have reviewed your code, it is looking good and I can't see any changes that need to be made. I will put it into second review.