Mechanistic vs Return Rate vs Rate: Need to decide on a single name for this

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Have been using 3 names interchangeably: "mechanistic", "return Rate", and "rate".

In the package functions and app we need to avoid confusion and messiness, so have to pick one.

I'm currently implementing "rate", just because that's what I used in the code of the estimation function.

Not a big deal to change, though.

So, just need a team decision, and then stick to it :-)

[SOLV-Code]

Executive decision: call it ReturnRate

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Done