TODO for version 1.0
- Think about what a user would expect from version 1.0
- Complete the main package vignette named mkin to include a method description
- Improve order of parameters in output

Nice to have:
- Get starting values for formation fractions from data
- Calculate confidence intervals for more than one formation fraction using Monte Carlo simulations
- 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