PDFs in GUI not fully descriptive

This issue documents that the PDF buttons in the GUI are not completely descriptive, i.e. there is ambiguity around how `HISTOGRAM_PLEVEL_FIELD_SEQUENCE=False` and `HISTOGRAM_PLEVEL_FIELD_AGGREGATION` option4 being True differs.

One might be a histogram of all points together, the other might be the average histogram using the field method defined higher up.

This issue proposes two courses of action

1. Check the logic and recipes so we are clear how both of these work and how they are unique.
2. Update the GUI with more information to make it clearer to the user how they are unique.

Note this also affects surface fields and model level fields, as well as pressure fields shown in the image.

Start by tracing the GUI PDF buttons and the recipes using HISTOGRAM_PLEVEL_FIELD_SEQUENCE and HISTOGRAM_PLEVEL_FIELD_AGGREGATION option4. Confirm how the two configurations differ, then update the GUI information so the distinction is clear for pressure, surface, and model-level fields.

Written by the indexing model from the issue text.