Household size for UBI calculations needs to be parameterized

[jdebacker]

Currently, the following parameters needed to compute household UBI amounts are hardcoded into parameters.py:

• ubi_num_017_mat
• ubi_num_1864_mat
• ubi_num_65p_mat

These parameters should be specified in default_parameters.json. Country-specific models should contain code to estimate these parameter values given country demographic data.