Controller methods are not bound in the base Backend class

[GDYendell]

Currently Controller methods are bound (using MethodType) in the EPICS and Tango backends, so that they are callable from any context with an implicit self argument. The base class should do this so that it doesn't have to be repeated in every child class, and to allow simpler, more generic testing.

Acceptance Criteria

• Be able to load a controller into Backend and call the methods available in its SingleMapping.