ModelingCmdVariant impl macro

[adamchalmers]

• Add a new method to ModelingCmdVariant trait, fn name() which returns the command's name.
• Add a macro for implementing ModelingCmdVariant
• Add a macro for creating OkModelingCmdResponse
• Add CONTRIBUTING.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Coverage        ?   25.79%           
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  Files           ?       28           
  Lines           ?     1407           
  Branches        ?        0           
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  Hits            ?      363           
  Misses          ?     1044           
  Partials        ?        0           

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unittests	25.79% <0.00%> (?)

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[gserena01]

This is super, super helpful! Approving now in case no changes are needed, but I'd love a response to the comments!

[gserena01]

Looks great! Thank you for making contributions so much simpler!