kDataCart should be reworked in order to get a >= 7 dim array (kx,ky,kz,cha,echo,sl,rep)

[aTrotier]

Right now, the slice and 3D partition are stored in the 3rd dimensions and a keyword is required to convert the kspace into multi-slice 2D or 3D k-space.

It will be easier to work with an array >= 7 dimensions kx,ky,kz,ch,echo,sl,rep