Revisit units of extended source fit tutorial

[israelmcmc]

https://github.com/cositools/cosipy/blob/develop/docs/tutorials/spectral_fits/extended_source_fit/diffuse_511_spectral_fit.ipynb

The tutorial says that the units of the K normalization are differential:
K = [0.00046, 0.0011, 0.0027, 4.5e-3]/u.cm/u.cm/u.s/u.keV

However, the code normalizes the spectrum based on the integral flux (assuming a 1 keV bin size)

cosipy/cosipy/threeml/custom_functions.py

Line 177 in 65706f7

dataFlux = dataFlux / sum(dataFlux) #normalized since the dat file has one point per keV

In addition, on #191 we're changing the definition of K to be unitless. For now, we changed the file in order to get the same numerical output, but we still need to fix this.