Option to error on missing Jinja2 variables

[DavidHuber-NOAA]

Description

This allows a user to specify if they want to allow missing variables in a jinja2 template. If allow_missing == False, a NameError will be raised with the name of the variable that is undefined. The default is True, which is the current behavior.

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

How Has This Been Tested?

• [x] pynorms
• [x] pytests

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes need updates to the documentation. I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] New and existing tests pass with my changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 47.93%. Comparing base (71f6b10) to head (df2875c). Report is 9 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #27 +/- ## =========================================== + Coverage 47.87% 47.93% +0.06% =========================================== Files 18 18 Lines 1650 1650 Branches 335 335 =========================================== + Hits 790 791 +1 + Misses 801 799 -2 - Partials 59 60 +1 ```

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