Must have:
- Some more testing with coupled models with FOMC, DFOP, HS and SFORB for parent
- Some testing with SFORB for metabolites

Nice to have:
- Calculate confidence intervals for parameters assuming normal distribution
- Calculate confidence intervals for DT50 and DT90 values when only one parameter is involved
- Add more unit tests for mkinfit
- Document validation against fits documented in chapter 13 of FOCUS (2006)
- Reproduce example anaylses (L1, L2, ...) in FOCUS (2006)
- Generate confidence intervals for DT50 and DT90 values for other cases