Improving NiftiImage fetch semantics

[vijayiyer05]

Per comment, the two-step of first fetch and then getting data for NiftiImage objects retrieved from map objects may feel strange to users.

Possible improvement:

• Create a dependent property called NiftiImageInfo that does what fetch currently does
• Replace fetch method with a getNiftiImage method which actually gets the image data

[dickscheid]

This is a bit tricky, since in siibra fetch is always the step "outside" siibra, ie. going from a siibra object to a non-siibra (but highly interoperable) data type like NIfTI or pandas DataFrame. To be discussed further.