Must have:
- Fix remaining problem with the schaefer complex test data (test.R)
- Adapt mkinplot function

Nice to have:
- Make unit tests for mkinpredict, comparing different solution types
- Calculate confidence intervals for parameters assuming normal distribution
- Calculate confidence intervals for DT50 and DT90 values when only one parameter is involved
- Add 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