slice_selective_sinc_pulse Rewinder Bug

slice_selective_sinc_pulse allows the user to define the slice normal vector, which is used when generating the slice select gradient.

However when calculating the rewinder gradient, the following is used for the area

area=np.abs(ssgrad.area[2]/2)

which only considers the area of the last direction, which may be zero or incorrect.

Proposed fix would be

area=np.sum(np.abs(ssgrad.area/2))

I think this makes sense...