SEGYExport won't export datasets where Dimensions_012 is unavailable

This prevents SEGYExport from working correctly on 2D datasets or in other cases where Dimensions_01 is available but Dimensions_012 is not. It also won't work with crossline-oriented prestack or poststack seismic where the primary key corresponds to dimension 1 instead of dimension 2. We should identify which dimensions correspond to the sample dimension and always loop over the primary dimension. We should also read either from the 3D dimension group or the 2D dimension group that contains the first two dimensions of the 3D dimension group.