Add initialized and size to AMReX (#156)

* Add initizlized and size to AMReX

* Add doc strings

src/Base/AMReX.cpp

@@ -101,6 +101,13 @@ void init_AMReX(py::module& m)
           }, py::return_value_policy::reference,
           "Initialize AMReX library");
 
+    m.def("initialized", &Initialized,
+          "Returns true if there are any currently-active and initialized "
+          "AMReX instances (i.e. one for which amrex::Initialize has been called, "
+          "and amrex::Finalize has not). Otherwise false.");
+    m.def("size", &AMReX::size,
+          "The amr stack size, the number of amr instances pushed.");
+
     constexpr auto run_gc = []() {
         // explicitly run the garbage collector, so deleted objects
         // get freed.