Resolve "(core): image data fields final review"

Description

• MedicalImageSrv was simply removed.
• Instead, data::helper::MedicalImage::check*() functions are called when reading or creating an image. An image is "always" a medical image, but this does not hurt to have extra fields for non-medical images.
• checkLandmarks() was removed, the point list is now always present.
• The following used fields were removed: slices count, comment, label.

Closes #836 (closed)

How to test it?

• Run SightViewer and open an image with all the different image readers.
• Each time, observe the negato is centered at the middle of the image.

Data

• You need a VTK image, an inr.gz image (there are some in our test data, in igg/ folder) and a DICOM image.

Additional tests/tasks to run

• You can also run dicomXplorer and all tutorials affected by the removal of MedicalImageSrv.

Future work?

We are still not at the end of the job, but this is too big to fit in this incoming release. In Sight 22.0, we plan to convert all these fields as concrete image members. This will simplify the code even more.