Check input frames have same columns - missingness

[ADBond]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

Closes #808.

Give a brief description for the solution you have provided

The main thing here is that the property linker._input_columns now checks columns for all input dataframes, and gives a clear error message if they are not identical sets, outputting columns that are not common to all frames.

Other changes:

• renamed linker._get_input_columns -> linker._input_columns (clearer name for property vs method)
• linker._input_columns returns a list of InputColumns rather than just their names, for flexibility
  • and adjusted profile_data accordingly
• used linker._input_columns for missingness chart so we get the check for free
• fix missingness_chart docstring
• added simple tests

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter

[ThomasHepworth]

Ah, I didn't see Sam's PR from last month, but it duplicates some of the functionality within the settings validation class.

I think the linker object is a better place to house this code, but can we consolidate the two?

All I need on my end is the column_names_by_input_df variable.

[ThomasHepworth]

If you want to trim remove the quotes from our column names, there's some code in the settings validator to do this too. Though, I don't think this is a necessity.