JPAAbstractExpandQuery doesn`t fully support extension

[VMatyushevski]

The current design (especially modifiers) of JPAAbstractExpandQuery doesn`t fully support extension since the Class by itself is public, but the important methods that are used within an extention and other dependent classes have package private access.

PR Link to changes that might help to resolve: #371

[spatanjali]

@wog48 Would you please be able to share your thoughts on the above PR. We hope that the change will make it easier for the users of the project to extend certain features (such as pagination) via custom implementation.