[ENH] Improve complete function

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Simplify logic for complete, using pd.merge
• Simplify logic for expand_grid, which is also used in complete
• Returns MultiIndex column DataFrame for expand_grid
• Users can manipulate columns for expand_grid by collapse_levels or droplevel (power to the user)
• Speed improvement for complete for scenarios where reindex was previously used
• column order unchanged for complete
• use hypothesis as much as possible for testing (credits to @ericmjl )
• Add example notebook for complete

speed check (expand_grid) ... always with a pinch of salt

data = {'height': [60, 70], 'weight': [100, 140, 180], 'sex': ['Male', 'Female']}
ddata = {key : value*20 for key, value in data.items()}

# this PR
%timeit janitor.expand_grid(others = ddata).droplevel(1, 1)
13.6 ms ± 861 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

# dev
%timeit janitor.expand_grid(others = ddata)
12.6 ms ± 586 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

comparision with pd.merge, how='cross' :

df1 = pd.DataFrame({'left': ['foo', 'bar']})
df2 = pd.DataFrame({'right': [7, 8]})

 df1 = pd.concat([df1]*1_000)
df2 = pd.concat([df2]*1_000)

# PR
 %timeit janitor.expand_grid(others = {'df1':df1, 'df2':df2}).droplevel(0,1)
243 ms ± 22.6 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

# pd.merge
 %timeit df1.merge(df2, how = 'cross')
493 ms ± 65 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

This PR improves complete and expand_grid.

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

There will be automatic checks run on the PR. These include:

• 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 #933 (0177518) into dev (67535f0) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #933      +/-   ##
==========================================
+ Coverage   95.87%   95.90%   +0.03%     
==========================================
  Files          19       19              
  Lines        2618     2565      -53     
==========================================
- Hits         2510     2460      -50     
+ Misses        108      105       -3     

[ericmjl]

Wonderful stuff, thanks @samukweku!