Linearity tests in regression

[TomiSzucs]

We omitted tests of linearity in simple and multiple linear regression due to the following reasons:

• Comparing spearman and pearson correlations gives information on linearity
• If multivariate normality holds, only linear relationships are possible between the variables (see e.g., Vargha: Többváltozós statisztika dióhéjban, 2019)

This might not be apparent for the user, so it is worth considering to put this in the wiki.
Further confirmation on the link between mutlivariate normality and linearity is needed as well.