[FEAT] Reverse model converter

**Is your feature request related to a problem? Please describe.**
/ 

**Describe the solution you'd like**
Add a model converter converting a `gammapy` model to an `astromodels` model

**Describe alternatives you've considered**
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**Additional context**
Would be useful for already existing analysis using `gammapy` to simply convert it instead of modelling it from scratch using `astromodels`
