Update asset data

[jdebacker]

SOI now has published data through 2015 (e.g., partnership tables). BEA detailed fixed asset data is available through 2016.

If the format of the SOI data has remained the same, this should be a relatively straight-forward update.

[jdebacker]

@rkasher is now working on this in PR #217. SOI still has not released the 2015 corporate data.