amrex::Finalize() w/ gc.collect() (#83)

When calling `amrex.finalize()`, first call the Python
garbage collector. This is purely a bandage to make
usage like this easy:

```py
import amrex

amrex.initialize([])
mfab = amrex.MultiFab( ... )
del mfab
amrex.finalize()
```

Respect the Python standard guarantees without furhter headache,
by running an explicit call to the garbage collector.

Previously, this was only working with CPython, but alternative
implementations like PyPy might delay the true free due to time
based memory collection approaches.

src/Base/AMReX.cpp

@@ -103,7 +103,25 @@ void init_AMReX(py::module& m)
           }, py::return_value_policy::reference,
           "Initialize AMReX library");
 
+    constexpr auto run_gc = []() {
+        // explicitly run the garbage collector, so deleted objects
+        // get freed.
+        // This is a convenience helper/bandage for making work with Python
+        // garbage collectors in various implementations more easy.
+        // https://github.com/AMReX-Codes/pyamrex/issues/81
+        auto m_gc = py::module::import("gc");
+        auto collect = m_gc.attr("collect");
+        collect();
+    };
+
+    m.def("finalize",
+          [run_gc]() {
+              run_gc();
+              amrex::Finalize();
+          });
     m.def("finalize",
-          py::overload_cast<>(&Finalize));
-    m.def("finalize", py::overload_cast<AMReX*>(&Finalize));
+          [run_gc](AMReX* pamrex) {
+              run_gc();
+              amrex::Finalize(pamrex);
+          });
 }