Implement error handling in plot_* functions

[Enchufa2]

The packages dplyr, tidyr and ggplot2 are marked as suggested, because they are only used by plot_* functions, which are helper methods, just for exploratory purposes.

Instead of simply failing when these packages are missing, it would be better to print a nice error message explaining these things.

[bart6114]

Something like below?

loadPlotDependencies<-function(){
  packages<-
    c("ggplot2","dplyr", "tidyr", "aPackageIDontHave")

  for(pkg in packages){
    if(!suppressWarnings(require(pkg, character.only = TRUE))){
      stop(sprintf('simmer\'s plotting functions depend on missing package "%s". Please install the package using: install.packages("%s")', pkg, pkg, call.=FALSE))
    }
  } 

}

> loadPlotDependencies()
Loading required package: aPackageIDontHave
Error in loadPlotDependencies() : 
  simmer's plotting functions depend on missing package "aPackageIDontHave". Please install the package using: install.packages("aPackageIDontHave")

[Enchufa2]

I was thinking of something like this.