Pypartmc

- added an example notebook depicting interoperability with an external optical library (PyMieScatt) - kudos jcurtis2
- fix in process_simulation_output example (BC mass fraction plot)
- README cleanups (incl. badges for launching the above two notebooks)
- started phasing out nbviewer badges in favor of render-on-github ones (the latter display svg graphics correctly)

- Matlab snippets added to README (and hooked into the Fortran-Python-Julia cross test on CI) (300)
- CITATION.cff file added + other package metadata updates
- CMake: self-contained header check fixes + checks for submodule initialisation

kudos rwgk for help in workarounding the pybind11/Matlab issue (https://github.com/pybind/cmake_example/pull/164)!

- code cleanups (avoiding `delete[]` statements in favor of scope-triggered std::vector dtors)

- new features in`AeroState`: `masses`, `volumes`, `mixing_state` and `diameters` (all were properties) became methods accepting optional enumerable parameters: `include` and `exclude`
- new features in `AeroParticle`: `species_volume` and `species_mass` now accept both indices and names as parameters
- introduced CMake-generation of `{$CMAKE_BINARY_DIR}/include/aero_data_parameters.hpp` to pass Fortran parameter values to C++
- new submodule (to support C++ <optional> on older compilers): https://github.com/TartanLlama/optional
- added information on Apple ARM binary packaging and on the rationale for the list-of-single-entry-dictionaries pattern to the README

kudos jcurtis2 for the work on `AeroState` and `AeroParticle`!

- access to more information in `aero_particle` (`absorb_cross_sect`, `scatter_cross_sect`, `asymmetry`, `refract_shell`, `refract_core`, `n_orig_part`, `least_create_time`, `greatest_create_time`, `id`) and `aero_data` (`n_source`) - kudos jcurtis2!
- workaround compatibility issues with newer `setuptools_scm`
- require `CMake` 3.8 to understand `CXX17` flags - thanks leeanne2 for reporting!
- make `pytest` execute `rand_init` before each test, not just before a test session starts
- `README.md` additions & fixes - thanks zdaq12

- one more try at fixing PyPI release uploads