Fix 'bad_override: Method ive::DirectionalSector::read hides  but does not override ive::ReadWrite::read because some type qualifiers do  not match'.
Fix 'bad_override: Method ive::DirectionalSector::read hides
 but does not override ive::ReadWrite::read because some type qualifiers do
 not match'.

Reported by Coverity: CID 86119 and 269 others

tomasthoresen/osg,marchelbling/osg,jtorresfabra/osg,openscenegraph/osg,jtorresfabra/osg,jtorresfabra/osg,marchelbling/osg,marchelbling/osg,scrawl/osg,artoolkit/osg,artoolkit/osg,artoolkit/osg,tomasthoresen/osg,scrawl/osg,jtorresfabra/osg,openscenegraph/osg,openscenegraph/osg,artoolkit/osg,marchelbling/osg,scrawl/osg,tomasthoresen/osg,openscenegraph/osg,scrawl/osg,tomasthoresen/osg