[ENH] Conditional_join

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• for inner join, build dataframe directly, instead of relying on concat (usually faster)
• add fix for _range_indices to support dual column labels from df

This PR improves conditional_join.

PR Checklist

Please ensure that you have done the following:

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

2. If you're not on the contributors list, add yourself to AUTHORS.rst.

3. Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

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• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

Merging #956 (ec40c96) into dev (9c4dcf1) will increase coverage by 0.01%.
The diff coverage is 98.27%.

@@            Coverage Diff             @@
##              dev     #956      +/-   ##
==========================================
+ Coverage   96.29%   96.30%   +0.01%     
==========================================
  Files          77       77              
  Lines        2805     2817      +12     
==========================================
+ Hits         2701     2713      +12     
  Misses        104      104