users can now specify a list as follows to save select dates in a similar manner to the way final_timestep_* variables are saved.

Example config entry:

"COMPARTMENT_SAVE_DATES": [
        "2021-01-11",
        "2022-04-11",
        "2022-05-11",
        "2023-12-11"
    ],

note all dates must be in YYYY-MM-DD format.

if the dates provided fall outside the period saved or occur before the INIT_DATE then they are safely ignored. By default the final_timestep variable is still always saved as before.

This PR also comes with a new method within AbstractDynodeRunner which saves all timesteps (including the final_timestep) called save_inference_timesteps(inferer)

All existing functionality remains the same, no changes are needed if users do not wish to save extra timesteps.

CLOSES #298