replace warning by message in `gen_varmod` when variable already exists

[timflutre]

Description

When the user requests to add variable(s) to the varmod file (argument var of the function gen_varmod), a warning is issued: "Variables bla, bla, and bla already in var.mod. Not repeating it.".

Expected behavior

Show a message instead of a warning.

Proposed solution (optional)

Add an argument verbose as in other functions, and only show the message if it is set to TRUE.