No unit tests and no validated math

[Xunie]

Create some unit tests that test correct functioning of the library.
Validate the correct functioning of the math through these unit tests.

[Xunie]

1150703
Just added CTest + a preliminary precision test. Still need to set test failure conditions, but I have to figure out what the maximum error of Kahan and Neumaier are. Perhaps we can keep a running estimate as to the max error on a summer. This lets users just sum what they want to and verify how close they might be to the true value!

Right now. Neumaier sometimes performance worse than Kahan. Which doesn't make a lot of sense to me, but whatever. We'll see how it goes once the test works and we have some better statistical testing implemented.