Move index calculations out of `StandardTomoLoader`

In particular, the following index calculation https://github.com/DiamondLightSource/httomo/blob/25dd6bff918fe10657df845b3930528d0813b1b6/httomo/runner/loader.py#L240 is doing the same sort of thing as this line in the reslice https://github.com/DiamondLightSource/httomo/blob/25dd6bff918fe10657df845b3930528d0813b1b6/httomo/data/hdf/_utils/reslice.py#L63

It would be sensible to collect the common calculation somewhere so both can make use of it, and to avoid going out of sync with one another in the future.