TODO for version 1.0
- Transfer (some?) unit tests to examples to make them more easily accessible
  for the user - they will be tested during R CMD check anyway
- Let mkin call mkin_wide_to_long automatically, if observed data are suitably defined
- Convert package vignettes to knitr and Rmd
- Think about what a user would expect from version 1.0
- Move the GUI to a separate package as it has special dependencies and should have it own versioning

Nice to have:
- Improve choice of starting parameters, in order to avoid failure of eigenvalue based solutions
  due to singular matrix
- Calculate confidence intervals for DT50 and DT90 values when only one parameter is involved
- Calculate transformation only DT50 values (exclude pathways to sink) as additional information
- Calculate DT50 values from smaller rate constant/eigenvalue of DFOP, HS and SFORB models
- Document assumptions and implications of fitting method used in FME in the package vignette