[GPU] GridBuilder instantiation

Change creation of the GridBuilder: Accordingly, to our discussions in the paper group, we want to get rid of the GridFactory.

Instead of having this:

auto gridFactory = GridFactory::make();
gridFactory->setTriangularMeshDiscretizationMethod(TriangularMeshDiscretizationMethod::POINT_IN_OBJECT);
auto gridBuilder = MultipleGridBuilder::makeShared(gridFactory);

we want this:

auto gridBuilder = MultipleGridBuilder::makeShared();