Fix one-sided gravity potential calculation

Commit 5112b8a misinterpreted the role of index 'i' and, as a result,
installed code that would only define one-sided gravity potentials of
the first cell (roughly)--albeit eventually using the geometry of the
last active cell.

This commit restores the original, intended behaviour.
Merge pull request #159 from bska/fix-gpot

Fix one-sided gravity potential calculation