Qpretrieve

- maintenance release

- ci: fix build pipeline

- feat: introduce scaling by cropping in Fourier space (2)
- fix: set padding value of gradient data to zero (not median)
- fix: amplitude retrieval not possible due to typo
- ref: change default padding for QLSI
- enh: introduce `fft_used` property in FFTFilter
- enh: allow filtering at the center of the Fourier domain
- setup: bump minimal Python requirement to 3.10
- docs: filter size for disk is radius not diameter
- docs: use png for example images (better quality)

- fix: performing QLSI without reference did not work anymore

- docs: added basic docs for user API

- enh: Cache Fourier transforms using weak references of the
input data (useful for the reference data)
- ref: QLSI background correction by division with reference
gradient data Hx and Hy (minor speed-up) (1)