Fix precedence of interval operator

The interval operator has a lower precedence than, for example, binary
operations. This was unexpected and incorrectly implemented in the
output functions. For now, this is fixed by enclosing intervals in
parentheses to avoid misinterpretations.

The existing unit tests are adjusted to the updated output format.

tests/TestSimplification.cpp

@@ -40,7 +40,7 @@ TEST_CASE("[simplification] Rules are simplified correctly", "[simplification]")
 		input << ":- not covered(I), I = 1..n.";
 		anthem::translate("input", input, context);
 
-		CHECK(output.str() == "((not covered(U1) and U1 in 1..n) -> #false)\n");
+		CHECK(output.str() == "((not covered(U1) and U1 in (1..n)) -> #false)\n");
 	}
 
 	SECTION("comparisons")