Added SpCellList unit tests and fixed getNeighbors

I fixed ealier unit tests where the grid was too small by adjusting
the cutoff distance.  I also added a unit test for SpCell::getNeighbors
based off of the test from ddMd::cell::getNeighbors.  After looking
running this test, it was clear that getNeighbors was double counting
pairs.  This was fixed by only returning atoms from cells with cellIds >=
this->id_.