Print to command window an interpretable output
of what is in a photoreceptors structure.

See also DefaultPhotoreceptors, FillInPhotoreceptors.

7/19/13 dhb Wrote it.
8/12/13 dhb Code more generally and get rid of some special cases.
dhb For cmf-like spectral functions, print out peak wavelengths and peak values.
10/16/13 mk fields() -> fieldnames() for Octave compatibility. Other
bug fixes, e.g., wrong use of ii for innermost for-loops.

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