[regression] polynomial regression model

[ludwigbothmann]

• make clearer how this can be estimated, i.e., how to set up a design matrix in order to frame it as a linear model
• include feature transformations as a concept (at the moment, the linear model may have multiple features, but these are considered to be original features of the data set)

[pkopper]

insert this between current slides 6/7 (7 in total) We can do it like here: https://compneuro.neuromatch.io/tutorials/W1D2_ModelFitting/student/W1D2_Tutorial4.html, formula 168 and also add an example of the transformations.