Include `pool_size` as hyperparameter to optimise over

[tallamjr]

Currently the pool_size configuration in some layers are set to the self.kernel_size as this was previously the same setting as 3 for both.

It might be worth investigating the effect of allowing this to vary and to be optimised for.

The code for adding something like this could look like:

        pool_size = trial.suggest_categorical("kernel_size", [3, 6, 12, 24, 48, 96])  # --> Pooling width
...
        model = SNXModel(
            num_classes=num_classes,
            kernel_size=kernel_size,
            pool_size=pool_size
        )

With modifications made to snX/layers.py file accordingly

[tallamjr]

This has now been included and can be closed with PR #61