Pysdm

cleanup and refactors incl. API changes:
- Core --> Particulator
- Core.particles --> Particulator.attributes

coalescence adaptivity on by default + minore fixes/cleanups

maintenance release to accompanying PySDM-examples v1.9 + one minor fix in pH calculation code

- TopHat spectrum + test
- Flatau-Walko-Cotton polynomial fit fot saturated vapour pressure wrt ice + test
- smoke tests for Alpert & Knopf 2016 Fig 1 example

- added Alpert & Knopf 2016 immersion freezing example
- cleanups

- new example based on Lowe et al. 2019 Nature Comm. paper on surface-active organics - kudos claresinger and rxward!
- kappa (times dry volume) as particle attribute, also optionally dry organic volume
- backend routines, new products, initialisation methods (spectro_glacial_sampling) and basic tests for upcoming immersion freezing dynamic
- VTK exporter metadata aligned to properly overlay netCDF output in Paraview
- 2D kinematic environment GUI improvements and major code cleanup
- UniformRandom spectral sampling
- cleanups, fixes, new tests (r_wet_init, condensation diagnotics, moments, ...)