SNR task: automatic pairing of images for subtraction method

[sophie22]

Is your feature request related to a problem? Please describe.
Current process requires user to create separate folders for each pair of image for SNR measurement. This is time-consuming.

Describe the solution you'd like
Images in a single folder could be 'paired up' based on the orientation in the Series Description tag, allowing for multiple sets of images to be in the same folder - processed at the same time, speeding up processing time and avoiding manual creation of additional folders/manual re-running of the SNR task on the subfolders.

Describe alternatives you've considered
Might be higher priority to implement this feature for the web-app, such that multiple image pairs can be uploaded and processed in bulk, without manually selecting each image file individually.

Open to suggestions. discussion around whether this would be a useful feature.

[sophie22]

similar to #57