Pypartmc

- made `GasState::mix_rats` a read-write property (previously was read-only) - kudos jcurtis2

- add getters for `RunPartOpt::t_max` and `RunPartOpt::del_t` (thanks jcurtis2)

- removed hardcoded `AeroState` init parameters
- added `AeroState::dist_sample`
- added `AeroState::__len__`
thanks jcurtis2!

- adding getter and setter for AeroMode::name

- reporting an error if no values present in time array (e.g., passed to Scenario ctor)
- making all C++ headers self-contained and implementation-free
- adding CMake checks to ensure the above holds throughout further development

- multi-mode `AeroDist` initialisation (also within `Scenario`s)
- `n_mode` property and `mode(idx)` accessor in `AeroDist`

thanks jcurtis2 for help!