Updating file extension for Excel files when exporting MGXS data

[pshriwise]

Description

As I was updating some of the notebook material to work with v0.14.0, I noticed that exporting MGXS/MDGXS data to an excel format with extension .xls no longer works b/c pandas deprecated support for that format sometime last year. Relevant pandas PR is here.

This simply updates the extension of excel files to .xlsx to comply with the change in pandas. I didn't add any tests here since the excel writing option is an optional dependency for pandas, but I'd be happy to do so if

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks!