Issue with isinstance check of Spectrum class

[Jason Neal]

Occuring when trying to subtract to spectra using the overloaded operators.
Occurs for all + - * / .
The incorrect isinstance call is causing it to skip to the end and not create a new_flux variable to assign in the new spectrum.

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-85-4c9c29792431> in <module>()
     11 print("instance", isinstance(moving_spec, sp))
     12 
---> 13 diff_spec = stationary_spec - moving_spec
     14 
     15 plt.plot(diff_spec.xaxis, diff_spec.flux)

/home/jneal/anaconda2/envs/py35/lib/python3.5/site-packages/spectrum_overload-0.1-py3.5.egg  spectrum_overload/Spectrum.py in __sub__(self, other)
    235             new_flux = self.flux - other
    236 
--> 237         return Spectrum(flux=new_flux, xaxis=self.xaxis, header=self.header,  calibrated=self.calibrated)
    238 
    239 

UnboundLocalError: local variable 'new_flux' referenced before assignment

[Jason Neal]

Need to see if this fixes the error where it was occuring still.