Clean up csv data and add date and age columns

I cleaned up the output.csv file, adding names for all columns and removing the first two rows which seemed misaligned.  Then I added three new columns using Pandas:

* Four-digit year of entry
* Age at hearing
* Age at hearing binned by decade (i.e. parolee was in 20s, 30s, etc at time of hearing)

The transforms used to create these columns are documented in Add date and age cols.ipynb - it also includes basic plots.