chore: Move pygix dependencies to an extras requirements set

[pbeaucage]

The PGGeneralIntegrator module introduces a dependency on pygix which is a fairly problematic package: not updated in 3 years, based on a really old PyFAI. In particular, it requires a pin on silx_kit==2.0.0 which is causing problems with other packages. I propose we move pygix and silx_kit==2.0.0 into an optional [grazing] extras spec and eventually explore pyFAI's built in package for grazing which is coming soon.