TST: Reduce cumulative risk test to stub.

Remove tests that were a copy of period risk behavior, to prepare
for adding cumulative risk specific tests.
TST: Improve answer key interface.

Instead of using the indexes defined in the answer key class
to index back into the answer key object, populate the answers
so that they are available as members of the answer key object.

Update period risk test to use new answer key structure.

Also, remove the rounding behavior from the answer sheet, leaving
the rounding to the consumer of the answer key values, so that
the values can be retrieved from the spreadsheet during answer
key __init__ without knowledge of the decimal point that the calling
code expects.
Correspondingly, change period risk tests to use
np.testing.assert_almost_equal when doing floating point comparison.