feat!(core): Change behaviour of detrend process

WHY IS THIS CHANGE NEEDED?:
- process currently first normalises by dividing by mean -- not strictly
  part of detrending
- process divides normalised time series by moving average -- difficult
  to justify as poor evidence for signal's trend being multiplicative to
  phenomenon of interest

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- remove normalising step
- subtract moving average instead of dividing

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- change is not backwards compatible
- does not affect aliby, agora, or the rest of postprocessor as none
  depend on detrend
- will affect skeletons/arin as modules & scripts rely on detrending as
  part of time series pre-processing -- but qualitatively little changes

EVIDENCE THAT COMMIT WORKS:
- testing on skeletons/arin/strain report script and inspecting output
  plots -- still makes sense

REFERENCES:
- issue #22