Document mobility covariates normalisation

[payoto]

These commits do not alter behaviours of the code, they document and functionalise the normalisation steps taken on the covariate arrays.

• 7fed945 is only comments and labels the steps

• 316641e functionalises the normalisation process

  tidyverse was also found to be missing from the conda env to run base-italy.r and is added.

I'd be very interested in any comments on the steps taken and their rationale as some of it is unclear to me.