Shapeout2

- enh: add button "Find best spacing" in Plot tab in Analysis View
that iteratively finds sane spacing values (110)
- enh: notify user in plot if contour spacing is too large (110)
- enh: add action to remove all datasets from a session (97)
- fix: mysterious behavior of subclassed pg.ColorBarItem (109)
- setup: bump dclab from 0.39.1 to 0.39.3 (increased verbosity,
renamed feature labels, refactored DCOR access)

- feat: add support for extensions in the form of loading dclab
plugin and machine-learning features (85)
- setup: drop support for Python 3.7
- setup: bump dclab from 0.37.2 to 0.39.1

- fix: wrong scale for viscosity in bulk emodulus computation (104)
- fix: save unused polygon filters in sessions (101)
- fix: allow to delete polygon filters (100)
- setup: bump dclab from 0.37.0 to 0.37.2 (enhanced speed for
operations involving contour computation)

- BREAKING CHANGE: The volume feature in all previous versions was
overestimated by about 2µm³. Please re-run your analysis pipeline.
- setup: bump pyqtgraph from 0.12.2 to 0.12.3
- setup: bump dclab from 0.36.1 to 0.37.0 (faulty volume computation)

- setup: bump dclab from 0.35.6 to 0.36.1 (improved export speed)

- build: remove console window (102)