Add jinja is list filter

[gshank]

resolves #200

Description

Create a Python method "is_list" and add it to our jinja filters for use in jinja code.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.28%. Comparing base (60ffb06) to head (8aa646b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
+ Coverage   67.37%   68.28%   +0.90%     
==========================================
  Files          52       52              
  Lines        3372     3374       +2     
==========================================
+ Hits         2272     2304      +32     
+ Misses       1100     1070      -30     

Flag	Coverage Δ
unit	68.28% <100.00%> (+0.90%)	⬆️

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