Pysdm

- new example: parcel simulation based on a setup from [Pyrcel documentation](https://pyrcel.readthedocs.io/en/latest/examples/basic_run.html) (kudos claresinger)
- adding dry option to ParticleVolumeVersusRadiusLogarithmSpectrum product
- arbitrary-moment product factory
- nbviewer badges in README.md
- cleanups

tada:
- Monte-Carlo super-particle-number-conserving collisional breakup representation
(original algorithm and implementation by edejong-caltech and jb-mackay)

misc:
- Lowe et al. 2019 (Pruppacher & Klett) diffusion kinetics/thermics & latent heat formula (thanks claresinger)
- Lowe 1977 saturation vapour pressure formulae (thanks claresinger)
- Murphy and Koop 2005 saturation vapour pressure formulae (thanks isilber)
- new product: FlowVelocityComponent
- new spectra: Gamma & Gaussian (thanks edejong-caltech)
- fixing race condition in coalescence counter increments (thanks jb-mackay)
- fixing non-rectangular domain handling in VTK exporter
- switch from SciPy to PySDM backend root-solver in CompressedFilmRuehl surface tension (thanks claresinger)
- swithing to use single buffer for all products (less memory allocated)
- better unit-test coverage for physics formulae incl. units (thanks claresinger)

- moving terminal velocity and coalescence kernels out of "physics"
(re-release to trigger DOI generation after enabling integration with Zenodo)

- new Szyszkowski-Langmuir surface tension model (and updates in Ruehl model) - kudos claresinger!
- JOSS paper branch merged into main one, added CI workflow to check the paper code
- new cooling rate attribute and product
- new max Courant number product
- VTK exporter fixes
- mass and heat accommodation coefficients alterable from within constants
- cleanups

- major refactor around physical constants handling (Formulae ctor now accepts a dictionary of constant values to use instead of the defaults)
- handling exdown -> pytest-codeblocks package name change in GA workflow files
- first smoke test for immersion freezing using 2d kinematic setup (both singular and time-dependent)
- binned terminal velocity product and a corresponding 2d kinematic GUI panel
- mixed-phase support at Moist environment base class level
- handling of non-spatial dimensions (e.g. histogram bins) in netCDF exporter
- cleanups

- module docstring coverage reached 100% (checked with pylint in CI)
- using `python -We -m pdoc` instead of `pdoc` to catch broken code links within docstring (and other issues)
- catching OSError when importing ThrustRTC and issuing a warning (`pdoc` parsing works then even on machines without CUDA)
- some minor code cleanups/refactors