Move code specific to a database engine to solentware-base project

[RogerMarsh]

While looking at solentware-base Issue 1 it became clear some code could easily be moved with the likely result all dpt related modules would disappear from this project.

The datasourcecursor modules in core and dpt are the ones affected.

The clear-out would extend to clients of this project.

[RogerMarsh]

Most of the core.datasourceset module is not implemented and it can be removed along with the dpt version.