Add function to get per-bodypart RMSE

[n-poulsen]

Adds a function to get RMSE for each bodypart, for both single and multi-animal projects.

• Default behavior does not change (does not evaluate per-bodypart)
• Evaluates per-bodypart when calling evaluate_network(..., per_keypoint_evaluation=True) and outputs the results to a file in the evaluation-results folder named DLCscorer + "-keypoint-results.csv"
• Tests the evaluation code using synthetic data

[MMathisLab]

Awesome, I think this is a really useful addition. Edit: great, covers maDLC and dlc, thanks!

[n-poulsen]

Sure, I'll add the parameter to the functional tests

[jeylau]

Looking great, thanks @n-poulsen!