bd CPS: new variables for earnings imputations

[bdecon]

Would be good to know when an earnings value is imputed. This is not available for all years, but is indicated by PRHERNAL for hourly and PRWERNAL for weekly.

[bdecon]

Proposal here:
New variable IMPUTED

• 'none' if no wage/hours imputation
• 'hours vary' if imputed hours
• 'hourly wage' if PRHERNAL, and
• 'weekly wage' if PRWERNAL