Change the demographic variables pulled from PSID

[prrathi]

Fixes #20, removes num_children and num_children_away_from_home, available for only 1 and 2 years of data respectively. Adds num_family with the total number of family members with data for nearly every year.

[jdebacker]

@prrathi Thanks for this PR. Can you use the variable search at the PSID cross-year index and screen shot the table with the years of availability with these variables?

Thanks!

[prrathi]

Here are num_children_away_from_home and num_children, respectively:

Here is num_family

[jdebacker]

Ok - looks good @prrathi. Merging now. Thanks!