Compute and update the state variables for
a cohort
     
    
    DTS_step_cohort(a, y, pars, F_eir)
 
     
    
    Arguments
    - a
 
age of a cohort
 
- y
 
state vector
 
- pars
 
a list
 
- F_eir
 
a trace function that returns the eir as a function of time
 
 
    
    Value
    
a vector containing the vector of all state derivatives