Add net_wealth to psid_data_setup.py

[MaxGhenis]

Per #15 (comment), this is blocked by floswald/psidR#41, which will allow the psidR to work with the PSID wealth supplements after their alteration with the 2017 PSID release.

[jdebacker]

Related, PR #55 adds a data file with the PSID through 2015 that includes the net wealth variable. Obviously, long term solution is to have an update to psidR to be able to read wealth data from latest PSID surveys, but this provides and option for now.