Move data_utils functions to virus-clade-utils

[bsweger]

Background

@rogersbw Has a repo that contains his work to generate a list of COVID clades that will be predicted for each round of the upcoming covid-variant-nowcast hub: https://github.com/rogersbw/clade_data_utils/

@elray1 and I discussed where the various pieces of the variant nowcast hub code should live and propose:

• data_prep and clades_to_model functions (data_utility.py) move to the virus-clade-utils repo

Definition of done

• [x] Add data_prep function to virus-clade-utils
• [x] Add clades_to_model function to virus-clade-utils
• [x] Corresponding unit tests