Interface "RestorableUniformRandomProvider"

In order to make usage of the "save/restore" functionality slightly more intuitive, a new interface will extend UniformRandomProvider with the following methods:

RandomSourceState saveState();
void restoreState(RandomSourceState state);

where "RandomSourceState" is a marker interface (whose concrete implementation is the RandomSource.State class).

Those methods will replace their counterpart currently defined in class RandomSource.