Terminology in code

[annayqho]

Make it consistent with the paper. From what I can tell, this is the final list.

• "stellar labels"
• "reference objects"
• "survey objects"
• "training step"
• "test step"
• "label transfer"

[davidwhogg]

Not only should the words match, the symbols should too -- like M, m, N, n, theta, ell, etc...