`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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1. [x] PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

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3. [x] Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
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Relevant Reviewers

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• @ericmjl

[ericmjl]

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[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!