GRChombo examples/tests do not build with NAMESPACE=TRUE

[mirenradia]

Pretty much in the title. Since the default is NAMESPACE=FALSE, this is a problem which probably doesn't affect anyone but I have had issues trying to link with external libraries that have the same names of objects as those in Chombo. Building with NAMESPACE=TRUE removes this conflict as it places pretty much all of the Chombo code into a Chombo namespace. Besides, it is good C++ practice to keep things in namespaces rather than at the global scope.