Make jfactor a parameter for DarkMatter models

[StFroese]

It is common in dark matter analyses to have a prior on the J-Factor (see here).
In order to achieve this, the jfactor of the DarkMatterAnnihilationSpectralModel has to become a parameter.

I also renamed the J-Factor to D-Factor for the DarkMatterDecaySpectralModel and did the same there, this is also very common :)

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (1474bb9) 75.04% compared to head (cac65e8) 75.21%.
Report is 86 commits behind head on main.

Files	Patch %	Lines
gammapy/astro/darkmatter/spectra.py	37.50%	10 Missing ⚠️
gammapy/astro/darkmatter/tests/test_spectra.py	0.00%	5 Missing ⚠️
gammapy/modeling/parameter.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5097      +/-   ##
==========================================
+ Coverage   75.04%   75.21%   +0.17%     
==========================================
  Files         233      234       +1     
  Lines       34622    34813     +191     
==========================================
+ Hits        25982    26186     +204     
+ Misses       8640     8627      -13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[StFroese]

Maybe the jfactor should be given as log10(J) to avoid large values

[StFroese]

Maybe the jfactor should be given as log10(J) to avoid large values

Or a scale factor which is optimized instead.
$\langle \sigma \nu \rangle$ is also maximally degenerate to the J-Factor, this would make to linear scale factors to be optimised.
Any opinions on how to do this nicely?