shallowWaterFoam: Read hU rather than U to support complex e.g. time-varying BCs

Disadvantage is that the BC values have to be specified in terms of hU
rather than U.  The alternative would be to add complex code to map h
and U BCs into the equivalent for hU.
Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1566