[AGIPD@HED] [CORRECT] read acq_rate from slow data

A fix for read acq_rate (repetition rate) from the slow data instead of only fast data as it is alread implemented for the dark notebook.

Description

How Has This Been Tested?

Tested with /gpfs/exfel/exp/HED/202131/p900228/raw/r0086 which has 2.2 acq_rate.

Relevant Documents (optional)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

Reviewers

@jsztuk @schmidtp @danilevc