optim(data-viz): avoid rendering plot 2 times when var type is changed

Previoulsy, `data_from_file` depended on both `input$categ_variables`
and `input$quant_variables`. This cause the plot to be calculated 2
times because when the user change a variable "type", eg. `Var_1` from
"categorical" to "quantitative" the following happens:
 - `data_from_file` is recalculated because `input$categ_variables`
 had changed
 - an observer update the value of `input$quant_variables`
 - `data_from_file` is again recalculated because `input$quant_variables`
 had changed

Now I have removed this dependency since all variables that are not
categorical are quantitative.

Note: the `input$quant_variables` input is not actually used anymore
and is here only as information for users.

src/server/server_data_viz.R

@@ -99,22 +99,22 @@ observeEvent(input$categ_variables, {
   data <- req(raw_data_from_file())
   categ_var <- input$categ_variables
   quant_var <- colnames(data)[!colnames(data) %in% categ_var]
-  if (!setequal(quant_var, input$quant_variables)) {
-    updateSelectInput(session, "quant_variables",
-      selected = quant_var
-    )
-  }
-
+  updateSelectInput(session, "quant_variables",
+    selected = quant_var
+  )
 })
 
 
 data_from_file <- reactive({
 
   data <- raw_data_from_file()
-  for (var in input$quant_variables) {
+  categ_vars <- input$categ_variables
+  quant_vars <- colnames(data)[!colnames(data) %in% categ_vars]
+
+  for (var in quant_vars) {
     data[, var] <- as.numeric(data[, var])
   }
-  for (var in input$categ_variables) {
+  for (var in categ_vars) {
     data[, var] <- as.character(data[, var])
   }
   data