Parsing functions should be static

[justusschock]

🚀 Feature
Parsing methods like Image._parse_affine, Image._parse_tensor and Image._read_and_check currently don't touch the class state (via self). As they only do input validation, they also shouldn't.

To prevent this entirely and especially making it easier to test these when they are overwritten, to me it makes sense to make those functions completely static.

What do you think @fepegar ?

Note: This probably doesn't matter when using the built-in image classes, but heavily impacts testability for custom classes.

[fepegar]

Hi, @justusschock. I agree it makes sense to make any methods not using self static.