RadioSpectrum1D

RadioSpectrum1D is a pretty light inheritor (child?) of Spectrum1D; it overrides the arithmetic operations (\_\_add\_\_, \_\_sub\_\_, etc) with functions that allow a threshold to be specified, and a reason is given in the docstring.  Also defined a 'shape' property and \_\_len\_\_ and \_\_repr\_\_ methods.  These might (all?) be considered for inclusion in the base class, but at least the overridden arithmetic operators are probably important for radio work.