Add 'get_summary_variables' function to SummaryVariables

[pipliggins]

Adds a function to return a dictionary of all the summary variables + cycle number from the SummaryVariables class

Fixes #4765

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.70%. Comparing base (b8877b4) to head (bb22d8a).
Report is 91 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4824   +/-   ##
========================================
  Coverage    98.70%   98.70%           
========================================
  Files          303      303           
  Lines        23312    23314    +2     
========================================
+ Hits         23009    23011    +2     
  Misses         303      303           

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