Fix value of Ewald shift
In all the short-ranged kernel flavours, sh_ewald is subtracted from
rinv, which have inconsistent dimensions. Fortunately, rcutoff is
often close to 1, and the inconsistent shifts often cancel in
practice, and energy differences computed on neighbour lists of the
same size will have the error cancel. The difference doesn't even
show up in the regressiontests, but would if we had a unit test
of a single interaction.
Fixes #2215
Change-Id: Ia2ea57f3bd9d521879783b207353d9d6f4ccb4a8