Error messages & package loading

[SimonDedman]

Diya: Error in FUN(X[[i]], ...) : only defined on a data frame with all numeric variables 3 env vars categorical. Changed to numeric. If data need to be numeric I need to reflect that in the documentation AOR create a converter.

improve how functions are checked/loaded (obviated if in CRAN?) see this from gbm: if (!requireNamespace("gbm")) { stop("you need to install the gbm package to run this function")