TODO for version 1.0
- Let mkin call mkin_wide_to_long automatically, if observed data are suitably defined
- 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
- Support model definitions without pathway to sink in combination with formation fractions
- Complete the main package vignette named mkin to include a method description

Nice to have:
- 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