Update bundle adjustment

[nmayorov]

I have done some improvements on this notebook:

1. Clarify definition of camera parameters
2. Add a comment about relation between images and cameras for this particular setup
3. Slightly reworked the core part
4. Replace self-written function for rotation transform with Rotation class

Link to the updated notebook file

https://github.com/scipy/scipy-cookbook/blob/9e136daac9df507db7bf114bf319e6ade9723c21/ipython/bundle_adjustment.ipynb

Fixes #44

[nmayorov]

@rgommers

If you have time / interest check this too, I believe it is a clear improvement and safe to merge.

But let's wait for the people from the issues to comment.

[hxu296]

That looks great! I was translating this notebook from scipy to PyTorch and the original Rodrigues vector to unit quaternion implementation introduced some numeric instabiliity that took me a while to hunt down. Your change will solve this issue.

The code that introduced numeric instability:

theta = np.linalg.norm(camera_params[:, :3], axis=1)[:, np.newaxis]
with np.errstate(invalid='ignore'):
    v = camera_params[:, :3] / theta
    v = np.nan_to_num(v)