Qpsphere

- fix: guess initial values for image method when edge detection
method fails

- maintenance release

- feat: allow to select focusing position for rytov, mie, and mie-avg

- feat: add convenience methods for computing the background
phase mask using the location of the fitted sphere

- BREAKING CHANGE in `qpsphere.imagefit.alg.match_phase`:
The keyword argument `verbose_out_prefix` has been replaced
by `verbose_h5path`. The dependency on matplotlib (and
the creation of many PNG files) has been removed in favor
of a single hdf5 file that allows to retrace the fitting
procedure.
- fix: `max_iter` was actually `max_iter + 1`
in imagefit.alg.match_phase
- tests: improve coverage
- docs: minor improvements

- maintenance release to test automated deploy to PyPI