ENH: improvements to channel combination

[bennyrowland]

Channel combination is currently done with a rank-1 SVD algorithm. In some cases receiver channels have correlated noise and this should be whitened before the channel combination is performed. In addition, significant improvements in channel weighting estimation can be gained by apodisation of the FID signals to suppress the noise after the signal has died away, and for CSI data by performing some spatial blurring across voxels on the assumption that the sensitivities are spatially smooth compared to the voxel size.