Adds resuming and saving model to sb3 example

[Ivan-267]

Adds the ability to save and load model for resuming training, run inference and save periodic checkpoints with CL arguments.

Limitations of this implementation:

• While sb3 will save logs if starting multiple runs with the same experiment name by adding e.g. expname_1, expname_2 etc. to the folder name, my current checkpoint saving implementation requires a unique experiment dir or name argument to be set and prompts the user to set a different folder if the checkpoint folder exists, this is to prevent overwriting checkpoints from a previous run with the same experiment dir/name.

For example, if using the default arguments (no exp dir or name specified), the logs will be saved to: .\logs\sb3\Experiment_1 and checkpoints will be saved to: .\logs\sb3\Experiment_checkpoints

• Resuming training does not keep track of previous timesteps in this implementation. So for example training with --timesteps=10_000 and saving a model, then resuming training with the same setting and saving the model again will train the model for 20_000 total timesteps, but 10_000 steps will be displayed in the console log after the final training run.

This is just a draft implementation, I welcome your feedback on whether it is sufficient as specified before starting changes to the documentation.

[Ivan-267]

Nice, looking good! I haven't had time to test it though.

Thanks for the review. I ran a quick test on it in which it worked, but I didn't test every use case in-depth.

[Ivan-267]

This is great! Maybe we could expose the show_window parameter, in other scripts I have this option as --viz. I think training can be faster if you don't render. (but obviously you can't see the agent's progress)

Otherwise LGTM feel free to merge.

Thank you for the review, I agree. Camera sensors may need --viz when used (I haven't tested them at all yet so I'm not sure about this, also whether they work better with a GPU, which I can't test yet), if they do we can consider adding a help note later to where the argument appears.

Will merge after all tests pass.