compute_sensitivity() and the wrapper
sensitivity_analysis() now use fixed Css data with column
orders explicitly set to match other functions. The column orders are
now enforced in the underlying functions get_fixed_age(),
get_fixed_obesity(), and
get_fixed_params().
Added set_population(), which allows for age and/or
obesity status to be set directly and not require simulation.
The simulate_* functions can now handle population sizes that vary across regions.