Add the third party package, Spectractor, to lsst_distrib.

Spectractor is the core spectroscopic reduction part of the AuxTel pipeline, and the package atmospec, being added via RFC-799, provides an interface between DM code and Spectractor.

Spectractor has several dependencies which are not yet in the stack, which would need to be added to our conda env. For Spectractor these are normally installed via pip, but all are available in conda, and I think are even all on the conda-forge channel now too.

We currently run a fork, such that we have full control over versioning and updates etc, but I'd hope to make a PR to upstream once we make the requisite changes to our fork such that the packaging mechanisms remain in sync.