breaking change: simplify loan_size_* to only one version per loan book

[jacobvjk]

relates to #257

Instead of requiring separate columns for loan_size_outstanding, loan_size_outstanding_currency, loan_size_credit_limit, and loan_size_credit_limit_currency, we could simplify to keeping only two columns: loan_size and loan_size_currency.

While the distinction between the existing ones is useful in the analysis and indeed both outstanding and credit limit variables are used in actual applications, the simplification would be helpful for the following reasons:

• the calculations in PACTA are the same regardless of how exactly the financial exposure is defined. simplifying to one version would lower maintenance burden and case-handling in downstream applications (e.g. P4S)
• we cannot control exactly which variable a user has in their loan books and if that maps to our categories
• our categories are actually not defined extremely precisely
• the user would still be able to run PACTA both on outstanding loans and their credit lines. they would simply need to switch out the values in the loan book

OPEN: we could even think about a mechanism that allows the user to point to the relevant column in their loan book, rather than having them replace the values in loan_size. Open for discussion.

[jacobvjk]

closing this because George did not approve of the idea