order of output in `three_tissue_response_dhollander16`

[matteofrigo]

The returned variables listed in the API documentation are not in the correct order.

Listed:

    Returns
    -------
    S0_wm : float,
        white matter S0 response value.
    TR2_wm_model : Dmipy TR2AnisotropicTissueResponseModel,
        ModelFree representation of white matter response.
    S0_gm : float,
        grey matter S0 response value.
    TR1_gm_model : Dmipy TR1IsotropicTissueResponseModel,
        ModelFree representation of grey matter response.
    S0_csf : float,
        csf S0 response value.
    TR1_csf_model : Dmipy TR1IsotropicTissueResponseModel,
        ModelFree representation of csf response.
    three_tissue_selection: array of size (x, y, z, 3),
        RGB mask of selected voxels used for white/grey matter and CSD.

Actual:

return ([S0_wm, S0_gm, S0_csf],
            [TR2_wm_model, TR1_gm_model, TR1_csf_model],
            three_tissue_selection)

[rutgerfick]

Well spotted! has been updated.