Comparing floating point numbers in tests : comparing difference instead of using ==

[pgrt]

Hello,

In the hypervolume tests, some double are compared using ==, which can lead to test failures as it is safer to compare them by checking that their difference is small.
In the attached patch I changed some of these tests so that the failing tests now pass, but there remains some other such tests to change I think.

Best,
Pierre

check_equality_floating.txt