sumApprox and meanApprox doctests are incorrect

The doctests for sumApprox and meanApprox methods test against an upper bound but not a lower bound. If there was a regression in the underlying code that caused things to go wrong the doctest may not fail. For example if sumApprox returned 0 the doctest would return -1 which is less than 0.05. Solution is to use the abs() function to test that the approximate answer is within 5% of the exact answer.