implement `expand` method for pandas

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• implement expand method for pandas DataFrame
• user can build DataFrame to compute missing values/gaps, instead of using complete (more power to the user)
• updates to expand_grid - MultiIndex columns are generated only if the key lengths of the dictionaries differ (maybe a tuple was used, or the length of tuples differ)
• expand_grid now uses np.indices for faster generation of indices, which is used to explode into a cartesian.
• df_key is deprecated. The columns of df will be used instead as keys.

This PR resolves #1293 .

PR Checklist

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3. [ ] 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]

I'll fix the failures after other PRs are ok.