Add study infrastructure to explore tool behavior

[wscott]

The desired use of this tool is similar to James Welch's 3PEAT scheme for i-orp.com. Basically, you rerun it each year and adjust the current spending as the projections change over time.

I need to add a testing mode to demonstrate that rerunning the tool with fixed returns gives the same results as the original plan at the beginning of the period. Then we can use that feature to simulate using the tool over some historic period in the past. By running simulations over all possible periods from the last 100 years we should be able to select reasonable rules of thumb about what values should be used for returns and inflation.

A good solution is one where the variation in spending level from one year to the next is as low as possible. We don't want to be giving up possible spending, but a safety margin is needed to minimize the adjustments that are needed during market downturns.

In the 3PEAT paper, a 100% stock investment strategy was assumed. A study framework like this can be used to investigate using a stock/bond mix of investments with yearly rebalancing. It would be interesting to see if there is a sweet spot that obtains much of the average gain from 100% stocks but reducing the variation from market downturns. Similarly, I can see if there is a way to optimally select stock/bond ratios by age rather than just following common wisdom.