Return an age pattern \(\omega(a)\), a function of the form $$\omega(a) = \frac{A(a+\tau)}{B+a+\tau}$$ where \(\tau\) is a shift so that \(\omega(0) > 0\) and \(A\) and \(B\) are shape parameters
Usage
# S3 method for class 'type2'
make_function(opts)