get_cell_mask for 3d images has to return the lowest slice

[tigloo123]

Will incorporate this in the peripheral_fractionV2 branch

[tigloo123]

adjust_cell_mask should probably be removed since it is now superfluous

[tigloo123]

@ben : please verify for the spot coordinates, does max z value correspond to the bottom slice? (and 0 at the top of the cell)

[tigloo123]

Reverting back to the adjust_cell_mask : this adjustment stands for the zero_level