CategoricalImputer constant strategy

[bjherger]

• Update CategoricalImputer terminology to be consistent with SimpleInputer
  • fixed_value to constant
  • mode to most_frequent
  • replacement to fill_value
• Update documentation to reference strategy='constant' strategy
• Update default fill_value to '?', consistent with SimpleInputer
• Update unittests to match above
• Unit test for default fill value

This could be a quick fix to refactoring the existing strategy='fixed_value' option, and updating documentation.

[dukebody]

Hi @bjherger ! Sounds nice and easy to implement? Would you be willing to do so? We can provide help with tests & code review.

[bjherger]

@dukebody Sounds perfect! I've begun reviewing the contributing.md document, and I'll get working on a first pass.

Please feel free to assign this task to me, if you guys do that for tracking.

[bjherger]

Updating scoping to include changing mode to most_frequent

[bjherger]

@dukebody I've followed the contributing guide, written the code, and submitted a PR (#168 ). What's the best way to get feedback on this PR?

[devforfu]

@bjherger I'll review the changes today if I can do it before others.

[bjherger]

Adding unittest to acceptance criteria

[devforfu]

Solved with #168.