Simplify: Preserve type for 0-x to -x simplification

Plus test case, fixes #34.

prism-tests/bugfixes/simplify-unary-minus-issue-34.prism

@@ -0,0 +1,11 @@
+// Test case for https://github.com/prismmodelchecker/prism/issues/34
+//   Expression simplification for 0-x does not preserve type
+//   Error during model building (explicit), simulation
+
+dtmc
+
+module M1
+   s : [0..1] init 0;
+
+   [] true -> 1/2:(s'=0) + 1/2:(s'=max(1,0-s));
+endmodule

prism-tests/bugfixes/simplify-unary-minus-issue-34.prism.args

@@ -0,0 +1,5 @@
+-explicit
+-hybrid
+-mtbdd
+-sparse
+-exact

prism-tests/bugfixes/simplify-unary-minus-issue-34.prism.props

@@ -0,0 +1,2 @@
+// RESULT: 1
+P=?[F s=1]

prism/src/parser/visitor/Simplify.java

@@ -105,15 +105,23 @@ public class Simplify extends ASTTraverseModify
 		case ExpressionBinaryOp.MINUS:
 			if (Expression.isInt(e.getOperand2()) && e.getOperand2().evaluateInt() == 0)
 				return e.getOperand1();
-			if (Expression.isInt(e.getOperand1()) && e.getOperand1().evaluateInt() == 0)
-				return new ExpressionUnaryOp(ExpressionUnaryOp.MINUS, e.getOperand2());
+			if (Expression.isInt(e.getOperand1()) && e.getOperand1().evaluateInt() == 0) {
+				ExpressionUnaryOp simplified = new ExpressionUnaryOp(ExpressionUnaryOp.MINUS, e.getOperand2());
+				// preserve type
+				simplified.setType(e.getType());
+				return simplified;
+			}
 			if (Expression.isDouble(e.getOperand2()) && e.getOperand2().evaluateDouble() == 0.0) {
 				// Need to be careful that type is preserved
 				e.getOperand1().setType(e.getType());
 				return e.getOperand1();
 			}
-			if (Expression.isDouble(e.getOperand1()) && e.getOperand1().evaluateDouble() == 0.0)
-				return new ExpressionUnaryOp(ExpressionUnaryOp.MINUS, e.getOperand2());
+			if (Expression.isDouble(e.getOperand1()) && e.getOperand1().evaluateDouble() == 0.0) {
+				ExpressionUnaryOp simplified = new ExpressionUnaryOp(ExpressionUnaryOp.MINUS, e.getOperand2());
+				// preserve type
+				simplified.setType(e.getType());
+				return simplified;
+			}
 			break;
 		case ExpressionBinaryOp.TIMES:
 			if (Expression.isInt(e.getOperand2()) && e.getOperand2().evaluateInt() == 1)