BUG/Design: input/output variable naming: use {var}_frame, not {var}_df for dataframe

[josef-pkt]

based on a complaint in a japanese twitter thread

https://www.statsmodels.org/stable/generated/statsmodels.stats.diagnostic.acorr_ljungbox.html

uses model_df for degrees of freedom, and return_df for dataframe

abbreviation df should be reserved for degrees of freedom in code,
(I also mostly avoid df for dataframe in notebooks)

for summary type results I converged on using frame, e.g. summary_frame
more general patsy (and maybe we, at least in datasets) use return_pandas

[tatamiya]

Thank you for finding my tweets!

As to the return_df case, I think return_dataframe or return_frame is better.