New names for package and models

[twallema]

For future endeavors, it may be good to change the package name into something more general than covid19model, as this implies the package is to be solely used for COVID-related modeling. An example name could be IDM for 'infectious disease models' (very open to criticism here ;) ). It would be good to have a set of base models like SIR, SEIR, SEIRS, SEIRDS preprogrammed in a folder basemodels and then have a specific folder for our covid19model. Such repo structure will make it easy for future users to define other modified variants of SIR/SEIR models for other diseases like influenza.