convert_excel_date, convert_matlab_date for Polars

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Add convert_excel_date, convert_matlab_date for polars
• deprecate convert_unix_date
• update convert_excel_date and convert_matlab_date source code for pandas (vectorized pandas operations)
• avoid dataframe mutation in pandas

This PR relates to #1352 .

PR Checklist

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Relevant Reviewers

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• @ericmjl

[ericmjl]

Approving! Just need resolution of merge conflicts and we can merge!

[samukweku]

@ericmjl I made some changes to #1363 ... open to feedback on a better way

[ericmjl]

@samukweku looks like we have some conflicts to resolve on this branch, and then we can merge!