Re-architect inheritance structure to reduce complexity

[nfelt14]

Description

It would greatly reduce the complexity of tm_devices if the interface protocol classes (PIDevice, TSPDevice, etc.) were implemented as mixins that could just be inherited by the family base classes, rather than a main part of the inheritance tree.

This would make the folder structure much more usable and reduce unneeded complexity.

Additional Information

Notes:

• Move SignalGenerator class to Mixin or remove entirely
  • most functionality can just live in the AWG or AFG classes
• Move PIDevice and TSPDevice to mixins
• move RESTAPI device to mixin
• device type subpackages (scope, SMU, AFG, AWG, etc.) should be directly under the drivers subpackage
• shared methods/functionality should be moved into mixin classes rather than added to the inheritance tree in the drivers subpackage

[nfelt14]

This will be resolved when v3.0 is released.