Add calculation of predictor effects

nestedLogit 0.3.2

This is a modest enhancement to the package, adding effect displays and smoothing some infelicities in the previously released version.

• new Effect method for "nestedLogit" objects to create effect displays.
• add Effect example to vignette

nestedLogit 0.3.1

• correct buglet in as.data.frame method when newdata has one column
• predict.nestedLogit() now includes the newdata data frame as an additional component (.data) in its result. Consequently, the newdata argument is no longer required in the as.data.frame method.