Core: strip CompositeValue API (#683).

BREAKING CHANGE

The `CompositeValue` accessor methods which take a `defaultValue` parameter are dropped.
In conjunction with on-demand child topic loading (introduced in DM 4.1) the notion of a default value become pointless.
To check weather a certain child topic(s) is already loaded, use CompositeValue's `has(childTypeUri)`.

Dropped methods in CompositeValue:

Accessors:
- `Topic getTopic(String childTypeUri, Topic defaultTopic)`
- `List<Topic> getTopics(String childTypeUri, List<Topic> defaultValue)`

Convenience accessors:
- `String getString(String childTypeUri, String defaultValue)`
- `int getInt(String childTypeUri, int defaultValue)`
- `long getLong(String childTypeUri, long defaultValue)`
- `double getDouble(String childTypeUri, double defaultValue)`
- `boolean getBoolean(String childTypeUri, boolean defaultValue)`
- `Object getObject(String childTypeUri, Object defaultValue)`
- `CompositeValue getCompositeValue(String childTypeUri, CompositeValue defaultValue)`

See #683.