Add `EXPECT_ITEMS_EQ` assertion.
Clean up the unit tests a little bit.
Disable warning on signed/unsigned comparison.
Improve readability of tests on K-nn search.
Improved readability of tests on radius search.
Fixed bugs in unit tests for K-nn search.
Fixed computation of angles.
Disable warning on signed/unsigned integer comparisons.
Fix test for radius search.

N.B.: FLANN is not robust w.r.t. numerical precision.