Allow household consumption preference parameters to vary by age

[jdebacker]

A user has noted that it could be helpful (e.g., in the case of modeling the consumption of healthcare services) to allow the parameters describing household preferences over differentiated consumption goods to vary by age (s). The two relevant parameters are alpha_c and c_min, both of which only vary over consumption good, i.

Changing the c_min parameter doesn't have far reaching implications for the computation of the model solution (it may be only that the household.get_cons and household.get_ci functions would need to be changed.

However, if alpha_c is allowed to vary by age, then the composite good price, $\tilde{p}t$ would vary by age, $\tilde{p}{s,t}$. This would not affect the equilibrium solution, but would need to be accounted for in several places of the computational solution.