FIX: Several fixes on the FV function

(1+x)^(1+y) was stringifyfied incorrectly
We still need work on this

FV now returns currency

FV(-1,-2,1) should return #DIV/0! not #NUM!

base/src/test/test_fn_formulatext.rs

@@ -2,9 +2,6 @@
 
 use crate::test::util::new_empty_model;
 
-#[test]
-fn simple_cases() {}
-
 #[test]
 fn wrong_number_of_arguments() {
     let mut model = new_empty_model();