- Implement terme fixe
- Lots more loadings and rebates
- fix some issues with premium and present value calculation
- Calculate total benefix present value (survival, death, premium refund)
- Loadings can be overridden in the contract object (passed to the tarif,
  where the tarif-wide and contract-specific loadings are merged and
  contract-specific settings take precedence)
- First attempt at reserve calcultion
- Move contract class to its own file
- Valuation Tables: add modification function to adjust resulting death
  probabilities (e.g. monotonize, set lower bound for q_x, etc.)
- Properly implement age-shifted tables, add ageShift(table, YOB) function