refactor: TypeDouble expressions may evaluate to BigRational, add proper casts

This allows storage of BigRational values in StateValues / Values vectors, e.g.,
to store constants that have been evaluated exactly.

TypeDouble.castValueTo now returns a Number instead of a Double, requiring the use
of the doubleValue() method in several places where the value is evaluated using
double arithmetic.

prism/src/explicit/StateValues.java

@@ -139,7 +139,7 @@ public class StateValues implements StateVector
 				valuesI[i] = initI;
 		} else if (type instanceof TypeDouble) {
 			valuesD = new double[size];
-			Double objD = ((TypeDouble) type).castValueTo(init);
+			Double objD = ((TypeDouble) type).castValueTo(init).doubleValue();
 			double initD = objD.doubleValue();
 			for (i = 0; i < size; i++)
 				valuesD[i] = initD;

prism/src/parser/Values.java

@@ -28,6 +28,7 @@ package parser;
 
 import java.util.ArrayList;
 
+import param.BigRational;
 import parser.type.Type;
 import parser.type.TypeBool;
 import parser.type.TypeDouble;
@@ -220,6 +221,7 @@ public class Values implements Cloneable //implements Comparable
 		Object o = values.get(i);
 		if (o instanceof Integer) return TypeInt.getInstance();
 		if (o instanceof Double)  return TypeDouble.getInstance();
+		if (o instanceof BigRational) return TypeDouble.getInstance();
 		if (o instanceof Boolean) return TypeBool.getInstance();
 		else return null;
 	}

prism/src/parser/ast/Expression.java

@@ -355,6 +355,9 @@ public abstract class Expression extends ASTElement
 		if (o instanceof Double) {
 			return ((Double) o).doubleValue();
 		}
+		if (o instanceof BigRational) {
+			return ((BigRational)o).doubleValue();
+		}
 		if (o instanceof Boolean) {
 			return ((Boolean) o).booleanValue() ? 1.0 : 0.0;
 		}

prism/src/parser/type/TypeDouble.java

@@ -26,6 +26,7 @@
 
 package parser.type;
 
+import param.BigRational;
 import prism.PrismLangException;
 
 public class TypeDouble extends Type 
@@ -70,10 +71,12 @@ public class TypeDouble extends Type
 	}
 	
 	@Override
-	public Double castValueTo(Object value) throws PrismLangException
+	public Number castValueTo(Object value) throws PrismLangException
 	{
 		if (value instanceof Double)
 			return (Double) value;
+		if (value instanceof BigRational)
+			return (BigRational) value;
 		if (value instanceof Integer)
 			return new Double(((Integer) value).intValue());
 		else