Automagically find the ACS

[Nicholas McKibben]

Given the assumption that the ACS region is a hyper-rectangle with origin at the center, we would like to be able to able to detect it inside of kspace instead of passing it separately in calib.

• Useful for Siemens data which comes as kspace + calib when twixread is used
• Without restriction that hyper-rectangle is at origin, this is a very hard problem with increasing dimensionality, so it would be reasonable to restrict to ndim<=3
• Some work already done for 2D case in utils

[Nicholas McKibben]

mdgrappa is the only function so far to reference this functionality, but throws NotImplementedError when calib=None