Use class decorators to register Expr subtypes instead of the old classes.py approach

[Martin Sandve Alnæs]

From python 2.6, class decorators are available. We can use them to register Expr subtypes and get rid of having to import all types in classes.py. That will perhaps make it easier to add new types from the outside.

[Martin Sandve Alnæs]

What we have now is close enough to close this issue. Future iterative improvements can get their own issue number.

[Martin Sandve Alnæs]

Removing milestone: 1.5 (automated comment)