Change the type of age variable to int

Currently, the type is float: https://github.com/PolicyEngine/policyengine-us/blob/master/policyengine_us/variables/household/demographic/age/age.py. This causes age to be displayed as a float in the app: https://github.com/PolicyEngine/policyengine-app/issues/928. On the other hand, float ages seem to be needed in variables like ccdf_age_group.

Start with policyengine_us/variables/household/demographic/age/age.py and review how the age variable is defined. Then inspect the ccdf_age_group relationship mentioned in the issue and the linked app issue for the display impact. Done means age is displayed as an integer while variables that need fractional ages continue to work.

Written by the indexing model from the issue text.