TODO for version 1.0
- Think about what a user would expect from version 1.0
- Support model definitions without pathway to sink in combination with
  formation fractions
- Complete the main package vignette named mkin to include a method description
- Calculate pseudoDT50 values as recommended by FOCUS
- Improve formatting of differential equations in the summary

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