Drymass

- fix: skip sphere analysis when bad model parameters are detected
- fix: allow to force-regenerate the sphere analysis images file by
deleting it prior to running "dm_analyze_sphere"
- fix: do not plot profile in sphere analysis when the center position
not within the image data area
- setup: bump qpsphere from 0.5.3 to 0.5.4
- tests: fix failed test due to previous refactorization

- feat: allow to change the thresholding filter or use a manual threshold
- feat: new thresholding filter "dm-nuclei" designed for HL60 cell
nuclei segmentation
- fix: configuration file was "touched" even though it was only read
- setup: bump qpsphere from 0.5.2 to 0.5.3
- ref: increase verbosity of error messages when edge detection step
fails
- ref: new `threshold` submodule for all thresholding operations

- setup: bump qpformat from 0.10.2 to 0.10.3 (changes dataset identifiers)
- setup: bump qpsphere from 0.5.1 to 0.5.2

- setup: pin numpy<1.16.0 because of pinned scikit-image==0.14.0

- docs: minor improvements
- setup: pin scikit-image version again to 0.14.0 until the threshold_li
issue is resolved once and for all

- docs: minor update