reshape_niimg() may be too flexible

[tsalo]

Summary

reshape_niimg is more flexible than documented. reshape_niimg will sometimes take as input a file name, sometimes a 3D image array, and sometimes a 1D image array. It seems to work for all those conditions.

If it's a file name, it loads the file and then reshapes and if it's already a loaded image, it just uses it. If it's already 1D, it squeezes the file and is still 1D.

Additional Detail

Stems from #1060 (comment).

[handwerkerd]

From our April 2024 dev call, the decision is that the function has fine behavior, but we should update the document to accurately reflect what the function can do.