Compute the variance inflation factors (VIFs) for covariates in one level of the 
model (i.e., occupancy or detection). Calculation of VIFs follows the approach 
of function vif in package car, using the correlation matrix of 
fitted model parameters.
vif(mod, type)A named vector of variance inflation factor values for each covariate.
An unmarked fit object.
Level of the model for which to calculate VIFs (for example,
              'state')