Added check for domain decomposition and shells.

Fixes #1376 in as much as that it prevents incorrect calculations.
Can be left out when merging to 5.0 that does not support
particle decomposition. In 5.0 we can write a message about OpenMP
if that works with shells.
Change-Id: Ic07ae70cd3cd5298b6b2d579719da4d267b2d8fb