[parent] Re-export more Cargo features from pixeldata

[Enet4]

This exposes Cargo features image and ndarray from dicom-pixeldata to the parent crate. This will make it possible to use pixel data image and ndarray conversion capabilities through dicom like the example TOML below, hopefully preventing confusions similar to #465.

dicom = { version = "0.7", features = ["image", "ndarray"] }