Validate dark matter analysis

[morcuended]

Add a validation for dark matter analysis. Need more details.

Related to gammapy/gammapy#6102 and gammapy/gammapy#5097

Input from Aritra Gupta, in developers code, it seems that there are some discrepancies on calculations with other codes.

[aritra-gupta]

We can check the results obtained using the dark matter utilities in GammaPy with the following two standard references in dark matter phenomenology:

1. PPPC4. Specifically, we should be able to reproduce
   (a) Table (2) of this reference
   (b) Reproduction of Fig. (15)
   (c) The angular separation
   (d) The line of sight integral
   (e) The flux at source $\dfrac{dN}{dE}$ (Checking against the interpolated values from Pythia is enough).

2. MicrOmegAs. We can
   (a) recheck the line of sight integrals Eq.(7)
   (b) Tally against in-built popular theoretical models (Fig.(3))

3. Additional utilities to add to the existing modules:
   (a) Gamma Rays from Inverse Compton Scattering
   (b) Gamma Rays from synchroton radiation of e+e- which in turn were produced from dark matter.
   (c) Extra-Galactic Gamma Ray fluxes (Unlike galactic component, the computation depends on redshift $z$ and the dependence of optical depth of photon on it)