Must have:
- Some more tests with coupled models with FOMC, DFOP, HS and SFORB for parent
- Some tests with SFORB for metabolites
- Make the "method" argument work again - currently it is being passed both to ode() and modFit()
- Check the contents of the parms.ini argument to mkinfit for arguments that
  are not used in the model in order to avoid erroneous chi2 error level
  calculations

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
- Document assumptions and implications of fitting method used in FME in the package vignette