Pypartmc

- 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!

- added interfaces to `run_part_timestep` and `run_part_timeblock` (jcurtis2)
- added `init_env_state` to `Scenario` (jcurtis2)
- added `elapsed_time` and `start_time` to `EnvState` (jcurtis2)
- more initilisation logic within `run_part_opt` ctor (temporarily copy-pasted from PartMC)
- more meaningful CMake message when submodules not initialised
- fixed a few assert statements and added DEBUG builds to CI