RF: remove PARREC get_voxel_size method

``get_voxel_size` only being used in ``_calc_zooms`` method; fold code
into that method and delete.
BF: fix typo in PARREC general header field name
RF+TST: refactor / test ``_get_unique_image_prop``

I think the previous code was incorrect comparing 2D arrays, and I found
it hard to follow.  Refactor to do simple diff to find any differing
rows in the array; return scalar if selected array is 1D.