Draft API for assembly blocks

[adamchalmers]

Assembly block MVP needs 3 things:

• Moving bodies into the assembly
• Creating mate connectors
• Applying constraints to mate connectors

One nice-to-have that I've called out in a TODO:

• Translate/rotate calls where the engine persists its solutions into the KCL AST, ensuring warm solves.
• I don't know how the engine would send this back to KCL -- presumably a translate + rotate for each body in solved system, but what coordinate system would it be in? Scene global, or relative to something? Currently KCL translate has two coordinate frames available, the global scene, and the body's sketch's local system.

[codecov]

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[adamchalmers]

We could add options here to reorient the mate frame, e.g. translating it, rotating it, flipping the major/minor axes. WDYT David?