Harmonization of linear operator

[aTrotier]

Hi,
As discuss in the other topic in order to reuse the works of other package, it might be great to harmonize the linear operator package use.

The main advantages might be to be able to use multidimensional input/output rather than vector. I guess a first step might be to agreed about a standard multidimensional array. For example in BART :

enum mri_dims {
	READ_DIM,
	PHS1_DIM,
	PHS2_DIM,
	COIL_DIM,
	MAPS_DIM,
	TE_DIM,
	COEFF_DIM,
	COEFF2_DIM,
	ITER_DIM,
	CSHIFT_DIM,
	TIME_DIM,
	TIME2_DIM,
	LEVEL_DIM,
	SLICE_DIM,
	AVG_DIM,
	BATCH_DIM,
};

Do you think we should try to harmonize (It might required a lot of works in multiple package) or just let the user perform the conversion if they want some advance reconstruction pipeline that required to use the MIRT.jl and MRIReco for example ?