Add model statements with functions around covariates for FOSR

[julia-wrobel]

You get an error when using the plot_shiny() function, 'Error in get(pred.list[i], fosr.obj$data)' when trying to apply a function to a covariate in a model statement, for example:

fit.fosr = bayes_fosr(cca~ sqrt(pasat), data = DTI).

fails because of sqrt(pasat).