Add LN2_CIRCSIFT

[ofgulban]

Add circular shift along a chosen axis to some fix wrap around cases.

[ofgulban]

Example usage:

LN2CIRCSHIFT -input mydata.nii.gz -axis 2 -shift_neg 10

[ofgulban]

Second use case is using circular shift to fix echo shifts across z axis in multi echo imaging (this might be caused by thermal instabilities in the hardware). This can be thought as a very simple and constrained registration. In this example I have simply eyeballed the shift across echos in voxel units.