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/mod.rs

@@ -51,6 +51,7 @@ mod test_number_format;
 
 mod test_escape_quotes;
 mod test_extend;
+mod test_fn_fv;
 mod test_fn_type;
 mod test_frozen_rows_and_columns;
 mod test_geomean;