Abstract out the admin pension method

[chriskelly]

Turns out most pensions might work like admin's: 2% times max salary * years worked. While the rates applied to early/late years may vary, it should be a close enough estimate to just use admin's pension rates.

• Need to convert to age instead of year
• Need to calculate total years worked
• Perhaps allow defining of 2% retirement age in params
• When calculating spousal benefits in social security, worker's with a pension have to cut spousal benefit by 2/3 of pension payment https://www.ssa.gov/benefits/retirement/planner/gpo-calc.html