Remove unnecessary grid from fluid test.
Make all construction from deck use EclipseGrid.

The deck constructor is still present, but it will construct an
EclipseGrid object instead of doing its own equivalent processing.
Add PINCH support to grid construction.
Create pointer-to-const objects.

Older gcc (4.4) does not allow conversion of shared_ptr<Foo> to
shared_ptr<const Foo>. Pointed out by @bska.
Use new constructor for EclipseGrid.
Merge pull request #633 from atgeirr/use-pinch

Unify grid construction, allow PINCH