Support n-d svd_weighting

[bennyrowland]

The current svd_weighting code assumes that the input data is 2D, channels and ADC, but in some cases we might want to for example calculate the weights on repetitions x channels x ADC, which should be done by permuting the axes into the 2D shape channels x (repetitions x ADC) etc. This means svd_weighting() should support an optional axis parameter to define the axis of the provided ndarray which represents the channels.

[bennyrowland]

This was fixed by #132