Speed up centroid function (remove interpolation)

[jordiferrero]

Description of the change

I realised I had overengineered the com utils function. I have now simplified it with a huge speed improvement. It does not rely on scipy interpolation anymore.

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] docstring updated (if appropriate),
• [x] ready for review.