csv validation for start date, end date, and billing days

[ethanstrominger]

Acronyms Derived billing days: derived bd Provided billing days: provided bd

Proposal:

• If the start date, end date, and billing days are all provided,
  • ignore billing days, instead set billing days to end-start+1.
  • Note: currently, start date is set to end_date - start_date + 1, even if start date is provided
  • raise exception if -end_date - start_date not equal billing days or billing days - 1
    • start and end dates for billing records are not contiguous
• If the start date is not provided
  • continue setting start date = end date - billing_days + 1
  • raise exception if start date and end date are not contiguous