if a key is missing, report with a KeyError in AttrDict

[aerorahul]

Description

When a key is missing in a AttrDict, raise a KeyError consistent with dict.

Fixes #14

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• pynorms
• pytests

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.08%. Comparing base (dd9ca24) to head (91903a1).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #16      +/-   ##
===========================================
+ Coverage    48.87%   49.08%   +0.21%     
===========================================
  Files           15       15              
  Lines         1416     1416              
  Branches       308      308              
===========================================
+ Hits           692      695       +3     
+ Misses         670      667       -3     
  Partials        54       54              

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[AndrewEichmann-NOAA]

looks reasonable