Processing functions to add targeted rebates to bills

[eglucas]

This code adds two new functions in the summary.py module.

• transform_income_decile_elibiligity() to transform the ofgem archetype income decile tables to calculate the number of eligible households in the lowest X income deciles.
• subsidisation_table() to process baseline scenario results tables to produce another results table with the bill values for eligibile/ineligible households for a given eligibility criteria and rebate value. At present, I've left the output dataframe in pivot table format, but am open for it to be melted into tidy format if you think that would be most useful.

In this PR, could you please:

• Review the new functions code and flag any bugs, efficiency improvement opportunities, missing documentation.
• Check that the functions work as expected, you can do this in the new notebook I've set up rebates_targeting_development.py.

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Checklist:

• I have refactored my code out from notebooks/
• I have checked the code runs
• I have tested the code
• I have run pre-commit and addressed any issues not automatically fixed
• I have merged any new changes from dev
• I have documented the code
  • Major functions have docstrings
  • Appropriate information has been added to READMEs
• I have explained this PR above
• I have requested a code review