[AGIPD] Replace `agipdalgs.gain_choose_int` by fused type

Description

Noticed by chance while starting work on the LPD correction notebook that the gain_choose_int function is identical to gain_choose, but with a different input type. This can be neatly expressed with fused types in Cython, which duplicate the function on the C-level.

How Has This Been Tested?

Not yet, will do as part of LPD correction overhaul.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the code style of this project.

Reviewers

@ahmedk @kluyvert