Add support for properties of type Double

Fixes #14

Author: goneall

src/main/java/org/spdx/core/CoreModelObject.java

@@ -399,6 +399,25 @@ public Optional<Integer> getIntegerPropertyValue(PropertyDescriptor propertyDesc
 		}
 		return retval;
 	}
+
+	/**
+	 * @param propertyDescriptor Descriptor for a property
+	 * @return the Optional Double value associated with a property, null if no value is present
+	 * @throws InvalidSPDXAnalysisException on any SPDX related exception
+	 */
+	public Optional<Double> getDoublePropertyValue(PropertyDescriptor propertyDescriptor) throws InvalidSPDXAnalysisException {
+		Optional<Object> result = getObjectPropertyValue(propertyDescriptor, Double.class);
+		Optional<Double> retval;
+		if (result.isPresent()) {
+			if (!(result.get() instanceof Double)) {
+				throw new SpdxInvalidTypeException(PROPERTY_MSG+propertyDescriptor+" is not of type Double");
+			}
+			retval = Optional.of((Double)result.get());
+		} else {
+			retval = Optional.empty();
+		}
+		return retval;
+	}
 
 	/**
 	 * @param propertyDescriptor descriptor for the property

src/main/java/org/spdx/core/ModelObjectHelper.java

@@ -233,7 +233,8 @@ public static Object modelObjectToStoredObject(Object value, IModelStore modelSt
 			} else {
 				return mValue.toTypedValue();
 			}
-		} else if (value instanceof Integer || value instanceof String || value instanceof Boolean) {
+		} else if (value instanceof Integer || value instanceof String ||
+				value instanceof Boolean || value instanceof Double) {
 			return value;
 		} else if (Objects.isNull(value)) {
 			throw new SpdxInvalidTypeException("Property value is null");