Revisit Type Checking Throughout Project

[gbroques]

We do a lot of type checking throughout the project as there's a lot of code that needs to know this object is an axis, frame, extruder, heated bed, etc.

This seems like a cross-cutting concern, and we should probably have some type_check utility module or documented pattern for solving this problem.

Research

Look for usages of .Proxy.Type in code.

Summarizing what code needs to know about the "type" of objects in the document:

1. Module for getting objects from the user's selection
   • For preventing passing invalid objects like plain cubes to attachment functions.
2. Attachment functions
   • For validating objects passed to attachment function are as advertised.
3. and the function for building the cut list
   • For querying a document for the objects in it.