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 formatting of differential equations in the summary
- Improve order of parameters in output
- Write unit tests for mkinerrmin
- Calculate confidence intervals for more than one formation fraction using Monte Carlo simulations
- When checking for degrees of freedom for metabolites, check if their time zero value is fixed
  instead of checking if the observed value is zero (usually in regulatory kinetics it is set to
  zero anyway, but in the case of known impurities this may not be the case).

Nice to have:
- Get starting values for formation fractions from data
- 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