Added yearly cross-sectional employment statistics

[andrewbaxter439]

What

• This should add cross-sectional calculations of employment and unemployment rates
• Rates calculated as proportion of 16-64yos who are "EmployedOrSelfEmployed" or "NotEmployed" by year

Why

• adds additional validation statistic assessing employment rate stability
• adds additional output statistic
• closes Add calculation of cross-sectional employment statistics for working age adults #175

Reviewer queries

• @justin-ven - does this seem to make sense? tiny change with no outward effects; matches manual calculations. Brief sense-check would be great!

[Copilot]

Pull Request Overview

This PR adds yearly cross-sectional employment statistics by computing employment and unemployment proportions among working-age (16–64) individuals.

• Introduces two new fields: PropEmployed and PropUnemployed.
• Uses a new AgeGroupCSfilter to restrict calculations to the target age group.
• Computes mean values for employment and unemployment using CrossSection and MeanArrayFunction.

[andrewbaxter439]

Going to merge this one if alright @pbronka @justin-ven - vvv small additions with no effects outside of stats file and can be reverted if needs be. want to use this output for the validating of fixing employment rates

[justin-ven]

Sounds good to me