PR for #176: Updated SaveData.py to remove warnings about future deprecation

[liaochris]

In #176, following the generic solution in https://stackoverflow.com/a/78066237, we updated the code to remove the warning about future deprecation of features.

• In the temporary folder issue176_fillna_warning there is a notebook where you can run SaveData to ensure that the warning no longer appears.
• I will delete the temporary folder issue176_fillna_warning once you finish reviewing, before I merge. This temporary folder is necessary because without it, there's no way of testing whether the warning shows up (it's not something we can test for using unit tests)

[liaochris]

Updated from master in https://github.com/gslab-econ/gslab_python/commit/98a0d73c5c10ce377b736daf7448756d72bcf771