Further implement image loader

[kretep]

User Story

Technical debt

The image loader needs further implementation to enable easy access to the image data. Callers of image loader methods should not have to load the data themselves.

To Do

• Move dicom image reader to image load module
• Add function to load data (or at least cache the path) for scans + annotations for a specific patient. A reference (cache) to the reader should be kept to prevent reloading multiple times.
• Make accessor function to retrieve scans (as vtkImageData) for loaded patient.
• Make accessor function to retrieve the annotations (as an array of vtkImageData) for loaded patient. (Maybe also a specific annotation?)