Validate fields given in `preview` loader parameter

A field can be given that is not actually used by httomo, but will also not cause httomo to fail.

For example, `det_y` can be given (instead of the correct `detector_y`), and this will nto be picked up, and httomo will instead use the default value for the `detector_y` dimension (which is to select all the data in that dimension - most likely _not_ what was intended, given that `det_y` was specificallly provided).

Fields that are not used in the `preview` value should not be allowed, and an error of some sort should be raised to avoid confusion when an intended previewing isn't seen when httomo runs, purely from a typo in the pipeline.

Start by tracing how the preview loader parameter is parsed and validated, then compare supplied fields with the fields httomo actually uses. Done means an unknown field such as det_y raises an error instead of silently falling back to the default detector_y selection, while valid preview fields continue to work.

Written by the indexing model from the issue text.