CmdStan - generate-quantities - document how to use new data for prediction

[mitzimorris]

Summary:

Document requirements on matching parameters between data fitting model and standalone gq model and show how to use fitted_params and new data to do prediction

Description:

see discussion on discourse: https://discourse.mc-stan.org/t/generated-quantities-returns-error-mismatch-between-model-and-fitted-parameters-csv-file/17869/14 and CmdStan issue (closed): https://github.com/stan-dev/cmdstan/issues/927

Additional Information:

Provide any additional information here.

Current Version:

v2.18.0

[mitzimorris]

link to Stan User's guide - put examples from there into CmdStanPy jupyter notebooks