A VariableImportance object holds its measures by name in @measures.
Each measure states what it quantifies (@kind, for example split gain,
permutation, or coefficient), the cases it was computed on
(@computed_on), whether its values can be negative (@signed) or depend
on the units of the predictors (@scale_dependent), how it ranks
predictors (@direction), and what its values are for this model
(@description); @values holds the value for each predictor.
varimp_table combines the measures into one data.frame.