Unit test distance calculations

[glesica]

This would be a relatively pleasant part of the code to add unit tests since these calculations are deterministic. Also, we want to make sure the vectorized and unvectorized versions produce the same results.

Unit tests for the DistanceCalculator class (make the class abstract so that we can re-use the tests for the non-vectorized and AVX versions) (George)

Since we’re going to have three implementations of this (eventually) we might as well write one suite of simple unit tests since we can re-use them for all three