Update derive_vars_dt and derive_vars_dtm

[bundfussr]

• “format with day and month specified as 'dd-mm': e.g. "15-06" for the 15th of June,”: it should be ‘mm-dd’ and “06-15”
• Example “# Impute partial dates to 4th of june” must be updated because the dates are imputed to 6th of April.
• “new_vars_prefix="AST".” should be formatted as code (new_vars_prefix parameter)
• Add adam keyword for derive_vars_dtm
• Remove comment in function declaration (line 551, 666, and 668)
• The flag_imputation parameter(in derive_vars_dtm) should be case insensitive. Add a case_insensitive parameter to assert_character_scalar. If it is set to TRUE, the argument is converted to lowercase (for checking the values and returning the argument).