A FluxFrame is a specialised form of one-dimensional Frame which
represents various systems used to represent the signal level in an
observation. The particular coordinate system to be used is specified
by setting the FluxFrame's System attribute qualified, as necessary, by
other attributes such as the units, etc (see the description of the
System attribute for details).
All flux values are assumed to be measured at the same frequency or
wavelength (as given by the SpecVal attribute). Thus this class is
more appropriate for use with images rather than spectra.