Adding the ability of generating synthetic datasets to the COSI plugin in 3ML, to allow the computation of goodness of fits.

[ldigesu]

Currently, it is not possible to use the "GoodnessOfFit " and "LikelihoodRatioTest" classes of 3ML when fitting COSI spectra. This is because these two procedures internally handle the generation of synthetic datasets. The current COSI plugin in 3ML cannot generate synthetic datasets based on internal properties such as background, spectra, and response.
The call to the GoodnessOfFit methods returns the following error: "AttributeError: 'COSILike' object has no attribute 'get_simulated_dataset'"

The reference for this in the 3ML documentation is here: https://threeml.readthedocs.io/en/latest/notebooks/gof_lrt.html

Goodness of fit and likelihood ratio tests are often needed in spectral analysis to evaluate how well the data are fitted by a model or to compare different physical models.

[israelmcmc]

Thanks for opening this issue, @ldigesu!