auto_var_types: Automaticall assign var_types to the attributes
Description
If the user did not provide var_types, the function will preassume every variables to be quantitative
variables. If a variable's type is character but either the user or the automatic step said it's a quantitative
variable, the var_types attribute for that variable will be overwritten as categorical variable. At the same time,
a warning message will be printed on the screen.