Move response-model convolution to response module

[israelmcmc]

Currently the code it locations:

• In the RL Imaging code
• In COSILike: around here https://github.com/cositools/cosipy/blob/develop/cosipy/threeml/COSILike.py
• Source injector

This needs to be moved to the response module.

This includes creating a ExtendedSourceModel class, which should include the code in COSILike.

Once that's done, make the other modules call that function, and remove the duplicated code.

The SourceInjector should propagate these changes and allow for the injection or extended sources (and eventually polarized ones)