Function to check the input Covariate Relativities
Performs assertions on inputted relativities, will raise an error if any checks fail. Currently checks on the: necessary column names used in dataframe, required factors and levels based on inputted factors, inputted relativities being non-negative numbers.
check_relativity(factors, relativity)
factors: named list of vectors, containing the name of the covariates and associated factors in vector form.relativity: relativity dataframe as defined in relativity_template.