Workaround missing support for as.formula in R package maxlike version 0.1-5

Today I noticed that the predict function for maxlike didn't support models build with formulas from "as.formula". The error I got was:

Error in predict.maxlikeFit(model, data.frame(p), ...) : 
  at least 1 covariate in the formula is not in rasters.

I send in a pull request but in the mean time you can workaround this problem with the following wrapper for the maxlike function:

## workaround missing support for as.formula in maxlike
maxlike <- function(formula, ...) {
  m <- maxlike::maxlike(formula, ...)
  m$call$formula <- formula
  return(m)
}