Iterate amplitude "/itn" group

At the end of an iterative calculation, we wish to save the resulting complex amplitude in real space as itn_array. It may on occasion be desirable to save intermediate calculation results as well. The itn_struct information associated with these files is:

pix_x_meters: real space pixel size in the first dimension.

pix_y_meters: real space pixel size in the second dimension.

pix_z_meters: real space pixel size in the third dimension. This value must be present but is ignored for 2D arrays.

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photon_scaling ($s_{p}$): multiply itn_array by this factor to convert it into photons per pixel. Set to zero if unknown.

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iterate_count ($i$; uint32): counter of iteration number.