Add reference to updated answer key with benchmark variance cells,
and use the new cells as the reference for the benchmark variance
test.
The values changed from the original hardcoded values, due to the
change to close over close benchmarks.
So that the answer key does not onerous on the SCM repo size, add a
utility to download the answer key automatically.
Prevent re-download on every test suite run if the local answer key
matches the latest version.
The risk tests originally were based on a spread sheet, with the
results of returns etc copy and pasted into the `test_risk` module.
Include the spreadsheet and read the values directly using a Python
Excel spreadsheet library.
Eddie Hebert