[AGIPD][CORRECT] Add new implementation of slopes_ff_from_path override

Description

To aid in commissioning and characterization tasks, DET requested to revive the slopes_ff_from_file argument present from before the introduction of the CalibrationData (v1) API.

This MR is the first part adding a new implementation (now called slopes_ff_from_path to support both file and directories, more details below) for this. As referring to constants from arbitrary paths has the potential to break reproducibility, there will be two more parts following:

1. Enforce the use of --non-reproducible in xfel-calibrate when changing this parameter
2. Special commissioning for non-user proposals causing the webservice to include --non-reproducible in the argument list and piping output to scratch rather than proc

The slopes_ff_from_path parameter may point to a file or directory:

• If a file, multiple PDUs and calibration may be overriden from it. This is the local output mode of the current AGIPD FF notebook and implemented as legacy mode.

• If a directory, expect files named according to the local output of dark notebooks (const_{calibration}_{pdu}.h5) and override accordingly. This is requested by DET to be used going forward.

How Has This Been Tested?

in_folder = "/gpfs/exfel/exp/SPB/202405/p006880/raw"
run = 260
karabo_id = "SPB_DET_AGIPD1M-1"
karabo_id_control = "SPB_IRU_AGIPD1M1"
xray_gain = True

I copied corresponding CCVs to /gpfs/exfel/data/scratch/schmidtp/random/agipd_ff for testing.

Types of changes

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

Reviewers

@ahmedk @kluyvert