Add `fitButterworth`

Add the function fitButterworth to curvefit in order allow for a smoother integration of this quite often used functionality

changed milestone to %2.2.0

requested review from @luenensc

assigned to @schaefed

• Since fitting a highpass filter makes not that much sence for fitting data, id rather remove the 'FILTER_TYPE' parameter to keep the interface slim

• Since most people might be puzzled by the term Butterworth, id rather call the function fitLowPassFilter

added 2 commits

• 413bb157 - add function fitButterworth
• 13b3e068 - Merge branch 'butterworth' of https://git.ufz.de/rdm-software/saqc into butterworth

Compare with previous version

enabled an automatic merge when the pipeline for 13b3e068 succeeds

merged

mentioned in commit 953ff5b3

mentioned in commit 63b55c6d

mentioned in commit 684dc8a0

mentioned in commit 8f7a90e4