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/expressions/parser/tests/mod.rs

@@ -0,0 +1,6 @@
+mod test_general;
+mod test_issue_155;
+mod test_move_formula;
+mod test_ranges;
+mod test_stringify;
+mod test_tables;