Persist the dataframes without extension

[kirangadhave]

If the IDE extension is not loaded, the df_.... (see #8) variables fail.

There is no recovery from this failure unless the IDE extension is loaded. We can generate a pandas code snippet that recreates the original variable.

How we show the snippet will matter a lot. The straightforward way is to copy the snippet as a Python comment similar to the dataframe access code, like in #8.

Another ideal solution would be to store the generated code with a different mime/type in the cell metadata and show it when the cell execution throws an error due to an undefined variable.