plot3logit 2.2.0

Major changes

Now plot3logit depends on R v. 3.5 or later.
Now field3logit can read categorical trinomial models fitted through function vgam of package VGAM (included amongst suggested packages).
list objects can be passed to argument delta of field3logit for computing multiple fields (a multifield3logit object is returned).
Argument ncurves of function field3logit has been renamed to nstreams. No backward compatibility has been implemented.
Point estimates are passed to argument model of field3logit, matrix should have an attribute with the values the dependent variable may take. The attribute has been renamed from labs to levels. No backward compatibility has been implemented.
Added package generics amongst dependencies in section Imports.

Added dataset USvote2016.
Corrected bug on reading ordinal models from matrix.
Added S3 methods of generics tidy for class field3logit.
Added S3 methods of generics as.data.frame, as_tibble and tidy for class multifield3logit.
Made autoplot the S3 method of generic autoplot for field3logit objects
Added S3 method of generic label for class field3logit.
Added S3 method of generic label for class multifield3logit.
Added S3 method of generics [ and [<- for multifield3logit class.
Added the generic labels<- and the S3 methods for field3logit and multifield3logit class.

Added functions for computing the confidence regions of the covariate effects of categorical trilogit models and function for representing confidence regions both in standard an gg graphics.
Changed dependencies of the package both in section “Depends” (updated the version of ggtern) and in section “Imports” (added dplyr, forcats, grDevices, lifecycle, purrr, Rdpack, tibble, tidyr, tidyselect and removed reshape2).
Added the plot function for multifield3logit in standard graphics.
Updated the functions for reading the output of mlogit according to the upcoming new version of the package. The package plot3logit handles the output of both the current and the upcoming version of mlogit.
Completely redefined the output of methods as.data.frame and fortify for objects of class field3logit and multifield3logit.
Function plot3logit is now deprecated and not updated.
Partially redefined function stat_3logit and added function stat_field3logit which works like the previous version of stat_3logit.

Added methods coef and vcov for objects of class field3logit and multifield3logit.

Added zero element (NULL) in the sum of field3logit and multifield3logit.
Added attribute ordinal to S3 class field3logit and updated the S3 method print.

The level Long of variable irregulariry of dataset cross_1year has been changed to High.

Bug solved for as.data.frame.field3logit when a zero-coefficient model is passed to field3logit. The previous version was not able to return a data.frame correctly built. Functions stat_3logit, and TernaryField have been adapted so as to draw points instead of arrows.

Interface of fortify methods has been redefined by including argument data, so that now all fortify methods of the package are compliant with the interface of the generic.
field3logit now can read categories labels of the dependent variable from attribute labs when argument model is a matrix. Example in the help of package presentation has been updated.

CITATION file has been amended: the title of the package and other typos have been corrected.
Revision of the file README.md.

First release of the package plot3logit on CRAN!