I would like to update the version of the third party package SNCosmo in the simulations stack. Meanwhile, SNcosmo functionality has been split and some of it is moved into a cython package extinction. This is now a dependency for the usual SNCosmo package. So, including this will be useful for upgrading SNCosmo. 

Within the simulations stack, our functions replace the extinction calculations with `sims_photUtils` based extinction calculations. We will continue to use these, so no calculations will change on account of adding `extinction`. It will allow users to use the LSST simulations stack sncosmo and also simultaneously continue doing other sncosmo functions (like certain fitting functions) which simulations stack does not offer.

In terms of DM functionality, the effects are quite minor: this would occupy a spot in the lsst github repository and perhaps make it difficult for a future package with the same name. Additionally, it could need to be tested on Jenkins.