[Image normalization workflow]: parameter to indicate how many frames should be combined

[nvaytet]

In the image normalization workflow, we have:

• a dark run (camera shutter closed) that would typically have e.g. 5 frames
• an open beam run (camera open but no sample), typically also 5 frames
• a sample run with any number of frames

We will:

• sum the dark frames together (averaging out the noise)
• sum the open beam frames together (same reason)
• for the sample run, it could be that each frame represents one rotation (in the case of tomography), or it could be that 10 frames were recorded per rotation (also to average out noise).

Hence, we need to add an additional parameter on the workflow that says how many sample run frames should be combined for a single rotation.