Rewrite detrend process

Summary

Rewrite detrend process

What is the current bug behavior?

The detrend process code is difficult to understand, therefore, difficult to maintain and be sure that it does what it should.

Possible fixes

In https://git.ecdf.ed.ac.uk/swain-lab/aliby/postprocessor/-/blob/master/postprocessor/core/processes/detrend.py:

• Lots of 'God lines' in lines 20-24 and 70-77 that are difficult to understand. We should break down the computation process into human-understandable, discrete steps, preferably with comments.
• Use DataFrame.rolling()
• Use standard routines in numpy and scipy, e.g. numpy.ma.average -- better documented, used by many people, therefore more sure that they're doing the right thing.