Rename "behavior" to "auxiliary variable" across the codebase

[stes]

Related to this review comment, we have multiple parts of the codebase where we talk about "behavior" when we really mean an arbitrary aux variable.

Here is an overview: https://github.com/search?q=repo%3AAdaptiveMotorControlLab%2FCEBRA%20behavior&type=code

It would be good to rename this consistently across the codebase.

[MMathisLab]

Many of the flagged "behavior" in above are completely appropriate; I only want to flag/raise code-specific refs where we should say auxiliary variable and give a behavior as an example.

[stes]

I mean instances like here, in particular in the distributions:

[MMathisLab]

Yes, I agree there, I just wanted to point out many instances of "behavior" are fine ;)