New Release Issues

[ilan-gold]

1. Need a unified ModelContainer (and also likely a EstimatorGLM) class for accessing attributes/methods, even if their return types are different. Something akin to _EstimatorBase at the moment
   a. Implement any missing methods on said container like error_codes, niter, fisher_inv (all for a base ModelContianer class so needs base class implemented), features (comes from input data for Model)
2. Need to test diffxpy model fitting to ensure accuracy + correctness