TST: Add parser for date values from answer key.

Refactor the reading of values from the Excel spreadsheet so that
parsers are configurable by index.

Needed so that we can parse columns that have dates, in addition
to floats as previously.
TST: Add answer key indexes for cumulative risk metrics.

Add indexes for Sharpe, returns and other values needed for
reading answers for cumulative risk metrics.

Prepare for unit test and matching change of implementation.
DOC: Add annotations file for answer key.

Currently, just provide a way to render to some of the data extracted.
Intended to have more thorough documentation of the spreadsheet,
explaining derivation/calculations in each sheet and column.