Build testing/spheresecttest and use it to debug spheresect

Rewrote spheresect to be totally symmetric in its handling of x1, x2.
Then only at the end offset the result by half so f1, f2 run from
0 to 1, instead of -0.5 to 0.5. Also use only f1 and f2 values to
decide the sorting order. These together seem to fix all the inconsistencies.
With luck it will then stop the particle leakage.