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_fv.rs

@@ -0,0 +1,36 @@
+#![allow(clippy::unwrap_used)]
+
+use crate::test::util::new_empty_model;
+
+#[test]
+fn computation() {
+    let i2 = "=-C2*(1+D2)^E2-F2*((D2+1)*((1+D2)^E2-1))/D2";
+
+    let mut model = new_empty_model();
+    model._set("C2", "1");
+    model._set("D2", "2");
+    model._set("E2", "3");
+    model._set("F2", "4");
+
+    model._set("I2", i2);
+
+    model.evaluate();
+
+    assert_eq!(model._get_text("I2"), "-183");
+    assert_eq!(model._get_formula("I2"), i2);
+}
+
+#[test]
+fn format_as_currency() {
+    let mut model = new_empty_model();
+    model._set("C2", "1");
+    model._set("D2", "2");
+    model._set("E2", "3");
+    model._set("F2", "4");
+
+    model._set("I2", "=FV(D2,E2,F2,C2,1)");
+
+    model.evaluate();
+
+    assert_eq!(model._get_text("I2"), "-$183.00");
+}