Fix Fed Stats due to recent changes

[chesterxgchen]

1. fed_stats is broken due to the simulator changes, change the workspace directory in the notebooks and examples
2. for df_stats job, notice the new datasets has empty line at end, causing pandas to mistaken numerical data type to string type. We change the prepare_data.py to remove empty line
3. update df_stats notebook to add panda data table format
4. cleaned notebooks output

Types of changes

• [x] Non-breaking change (fix or new feature that would not break existing functionality).
• [ ] Breaking change (fix or new feature that would cause existing functionality to change).
• [ ] New tests added to cover the changes.
• [ ] Quick tests passed locally by running ./runtest.sh.
• [ ] In-line docstrings updated.
• [ ] Documentation updated.

[chesterxgchen]

/buld

[chesterxgchen]

/build

[chesterxgchen]

LGTM. Are there any changes needed in https://github.com/NVIDIA/NVFlare/blob/main/examples/hello-world/step-by-step/higgs/stats/tabular_stats.ipynb?

good point, I need to fix that as well. Let me merge this and fix that in separate PR