[AGIPD][CORRECT] Fix assigning a list to ff_constants

Description

This is a fix that was introduced in this MR !899 (comment 401039)

@esobolev has reported this bug in detail in the linked MR.

This returns a set. dark_constants, pc_constants are lists. In line 649 they are concatenated, that raises exception

How Has This Been Tested?

Relevant Documents (optional)

TypeError                                 Traceback (most recent call last)
<ipython-input-19-32ef92fb1091> in <module>
     15 modules.drop(mod)
     16
---> 17 warn_missing_constants = set(dark_constants + pc_constants + ff_constants)
     18 warn_missing_constants -= error_missing_constants
     19 warn_missing_constants -= set(calibrations)
TypeError: can only concatenate list (not "set") to list

Types of changes

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

Checklist:

Reviewers

@esobolev @kluyvert