functionInput is a shiny server function which controls
all aspects of the FlaccoFunctionInput UI Module. Will be
called with callModule.
functionInput(input, output, session, stringsAsFactors)[shiny-input]
shiny input variable for the specific UI module.
[shiny-output object]
shiny output variable for the specific UI module.
[shiny-session object]
shiny session variable for the specific UI module.
[logical(1)]
How should strings be treated internally?