added doc for envis_preprocessor.py

[bashirk]

Description

This PR adds inline documentation to the envis_preprocessor.py file.

Relevant Issue

Issue #269

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the NimbleEdge Styleguide

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[divyanshugit]

LGTM!! @ramesht007