150 right shape

[Lenoush]

answer to https://github.com/mind-inria/mri-nufft/issues/150

Basically, every time you call op and adj_op, make sure the input image is of the shape <XYZ>, which was provided during initialization.

• create a function that checks whether the shape of the image (in this case, the data) == self.shape (from init)
• call it when op
• test this function
• call it when adj_op

The CPU-based backend is handled differently