We stop using DatamodelGlue.h everywhere while we are at it. This is because the DatamodelGlue concept noticeably slows down compilation when the data model gets large. We still need a datamodel visitor for event sources, but shouldn't need it anywhere else.

This gets us out of the mess where user classes are required to have certain constructor arguments in order to forward them to the parent constructor.

This was a bad abstraction. The use of the templated visitor pattern obscured some very simple code. It is far better for users to copy-paste PodioFileReader and adapt for their needs.

This isn't a reusable abstraction or utility either.