AtomsFramework - Shared Table support

Initial support for sharing a database table between classes and subclasses now exists.

There are new XML/Attribute options at the Class level
- SharedTableField
- SharedTableValue

If SharedTableField is defined a shared database table is assumed. You do not need to provide a reference field back to the parent table (since
it's shared) and the class will inherit the key information from the parent.

It works for simple load/save test cases at this stage but is not heavily tested so keep your eyes open if you use it.

All existing superclass and inheritance stuff should still work as is.