Use `click.IntRange` type to catch integer values < 1 instead of manually checking `max_cpu_slices`

The `--max-cpu-slices` flag has type `click.INT` which allows for integers < 1: https://github.com/DiamondLightSource/httomo/blob/6af2e2eef2c432f68d474c55de282168af8878b1/httomo/cli.py#L86-L91

However, for the maximum number of CPU slices, a value < 1 doesn't make sense, and thus there is a bit of code manually checking if the given value is < 1: https://github.com/DiamondLightSource/httomo/blob/6af2e2eef2c432f68d474c55de282168af8878b1/httomo/cli.py#L200-L201

Click provides the type [`click.IntRange`](https://click.palletsprojects.com/en/8.1.x/options/#range-options) which allows for specifying a certain range. In particular, it can be used to provide a boundary on one side of the range, and have the other side of the range be unbounded. This would allow clipping the range of the max CPU slices to not be below 1, but still be anything above 1.

A similar case this has been used for already is in the `--frames-per-chunk` flag value, which can't be less than 0, but can be any integer from 0 and above: https://github.com/DiamondLightSource/httomo/blob/6af2e2eef2c432f68d474c55de282168af8878b1/httomo/cli.py#L137-L142