Data Features：stat summaries

[RoxanneXin]

feat: a draft for a known data frame; function groups are in another file. implementing issue #2

The 2nd file with function groups include some stat summary that I didn't really point out in my issue #2, such as R square, correlation coefficient, standard error, and F test. I added this since it's related to model evaluation, and make my plots more clearer.

All files are drafts; I'll update them later, espscially the function one.

[RoxanneXin]

Finally, I give up the plot part. Instead, I decide to split all features and calculate the stat summaries for each independent variable.