renaming for clarity & consistency

Refactor Merge Request (MR)

Description of Refactor

 - propRates  -> parentFactor 
 - Length -> Number 
 - add troubleshooting section in readme mentioning rm -rf build
 - renamed local variables that shadowed contract-attributes
 - make ALL params start with '_'

Related Issues

part of fathom/org#82 (closed)

Task Checklist

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