Combine normoblast and reticulocyte populations

We now treat these as a single population in the bone marrow, with a
lifespan of 4.5 days between normoblast production and maturation (in
the circulation) into a normocyte. This is consistent with the data in
Hillman (1969) and Koepke & Koepke (1986).

By reducing the delay between normoblast production and reticulocyte
release into the circulation, we see modest changes in steady-state
results, primarily for extremely low iRBC removal rates (delta_iR) that
result in substantial decreases in haemoglobin.