149 add statistical tests

[msorvoja]

Adds the following computations and statistical hypothesis tests:

• Compute correlation and covariance matrices
• Test normality: Shapiro-Wilk and Anderson-Darling tests
• Test independence: Chi-square test

It is assumed that missing values are handled before the function call and that any variable whose data type isn't float is a categorical variable.

[nmaarnio]

Remember to add the .md file. My comments mainly concern input types as I am thinking in which way this function would the most convenient to use. Feel free to disagree.