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
- Improve formatting of differential equations in the summary
- Rename formation fractions during transformation and backtransformation as there is no 
  one to one relationship

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