Sabelhaus & Song (2010) age profiles of income volatility

[Mv77]

This PR adds tools to produce age-profiles of income volatilities from Sabelhaus & Song (2010). It uses the original estimates from the paper, which were generously shared by John Sabelhaus.

The code is pretty much ready but I'm looking for feedback on:

• Where should the estimates used to produce the profiles go? They are a small enough set of coefficients that I just put them in the python script. Would it be better to have them in external csv files?
• The location of the files within HARK: I see these tools being used in multiple places and given the flexibility afforded by implementing a function with different specifications, I thought they deserved their own file. Do you agree? Would you prefer all of this to just be written at the bottom of ConsIndShockModel.py? Would this be considered a dataset and thus be better placed at HARK/datasets (re: @sbenthall 's comment in init_lifecycle should be replaced with a real life cycle calibration #592)?

I also need to better document the time units (#878).

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[Mv77]

This PR is now ready. I'd like to request review @sbenthall @llorracc .

I am still looking for feedback on the location and format of these tools.

Yet, I am very happy with the results. @llorracc I added smoothing using a polynomial by default as you requested, but kept the option to get the original results (see the example file).

I am particularly satisfied with the tests: they check the outputs against the coefficients from the original paper. Also, they check that smoothed versions are not more than 10% off from the original results.

I also added a README in the folder crediting John Sabelhaus.