convert `pi_estimator.R` to a general executable script

[tavareshugo]

As the learning objective of this lesson is not how to run R scripts non-interactively, we can convert the R script into a general executable that "hides" what it is under-the-hood.

• Add this at top of the analysis_scripts/pi_estimator.R (note: we hardcode the path to the system-installed Rscript as we don't want conda environments, etc., messing this up):

  #!/usr/bin/Rscript

• Make it executable: chmod +x analysis_scripts/pi_estimator.R

• Rename: mv pi_estimator.R pi_estimator

• Adjust scripts and materials to use analysis_scripts/pi_estimator instead of Rscript analysis_scripts/pi_estimator.R