Python: Provide a more useful summary for DataFrames in the Environment pane

[seeM]

We currently see, e.g:

[617 rows x 6 columns] pandas.core.frame.DataFrame

It might be a nice quick win to provide a more useful summary in place of that, or at the very least, to display pandas.DataFrame instead of pandas.core.frameDataFrame.

Somewhat related to #968 although that discusses dataframe columns.

[petetronic]

Note that we have some design discussions that need to be had first before we tackle 968, as we have some changes in mind for the environment pane, including supporting multi-line summaries.

[seeM]

Right. Might be worth waiting on this until we support multi-line summaries