Correct stats for R^2

[lanery]

This PR makes corrections to the regression analysis in notebook 2: notebooks/2_temporal-variation-in-motility-metrics.ipynb. In particular, the following changes were made:

• Regression analysis now reports both the (Pearson) correlation coefficient as well as R^2 (coefficient of determination).
  A section at the end of the notebook was added to explain how to interpret the regression analysis results.
• The visualization (Figure 4 in the pub) was corrected to show the regression for confinement ratio, mean curvilinear speed, and max sprint length.
• The CSV file results/figure-4_regression_stats.csv (Supplemental Table of the pub) was updated accordingly.

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[taraeb]

looks good