Add example for Python/R interoperability using mass univariate t-test

[aman-coder03]

Reference issue (if any)

closes #13721

What does this implement/fix?

adds a tutorial example that runs a 2 sample mass univariate t-test on Epochs data,.., first in python using scipy.stats then the same test in R via rpy2 showing that both give identical results. The goal is to show the users how to get data into R, run stats there, and then bring the results back into Python, as starting point for more complex R based workflows

Additional information

follows the approach suggested by @larsoner in the issue ...keeping it simple and focused on one clear use case rather than trying to build general purpose utility

[aman-coder03]

windows 3.13 pip pre failure is unrelated to this PR, it's a missing nightly wheel for statsmodels>=0.15.0.dev697 on the scientific python nightly wheels index, which affects other PRs too

[larsoner]

Being fixed in #13728