Wrap WarpX instance and start multi particle container

* Fix test Python_pass_mpi_comm
* Start wrapper for multiparticle container
* Add return policy

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Python/pywarpx/_libwarpx.py

@@ -782,7 +782,7 @@ def add_real_comp(self, species_name, pid_name, comm=True):
         comm           : bool
             Should the component be communicated
         '''
-
+        wx = self.libwarpx_so.WarpX.get_instance()
         self.libwarpx_so.warpx_addRealComp(
             ctypes.c_char_p(species_name.encode('utf-8')),
             ctypes.c_char_p(pid_name.encode('utf-8')), comm

Source/Python/CMakeLists.txt

@@ -17,6 +17,7 @@ foreach(D IN LISTS WarpX_DIMS)
           PRIVATE
             # pybind11
             WarpX.cpp
+            MultiParticleContainer.cpp
         )
     endif()
 endforeach()

Source/Python/MultiParticleContainer.cpp

@@ -0,0 +1,15 @@
+/* Copyright 2021-2022 The WarpX Community
+ *
+ * Authors: Axel Huebl, Remi Lehe
+ * License: BSD-3-Clause-LBNL
+ */
+#include "pyWarpX.H"
+
+#include <Particles/MultiParticleContainer.H>
+
+namespace py = pybind11;
+
+void init_MultiParticleContainer (py::module& m)
+{
+    py::class_<MultiParticleContainer> mpc(m, "MultiParticleContainer");
+}

Source/Python/pyWarpX.cpp

@@ -31,6 +31,7 @@ namespace py = pybind11;
 
 // forward declarations of exposed classes
 void init_WarpX(py::module&);
+void init_MultiParticleContainer (py::module& m);
 
 PYBIND11_MODULE(PYWARPX_MODULE_NAME, m) {
     // make sure AMReX types are known
@@ -53,6 +54,7 @@ PYBIND11_MODULE(PYWARPX_MODULE_NAME, m) {
     )pbdoc";
 
     // note: order from parent to child classes
+    init_MultiParticleContainer(m);
     init_WarpX(m);
 
     // API runtime version
@@ -113,6 +115,11 @@ PYBIND11_MODULE(PYWARPX_MODULE_NAME, m) {
     m.def("check_gridding_for_RZ_spectral", &CheckGriddingForRZSpectral,
         "Ensure that the grid is setup appropriately with using the RZ spectral solver");
 
+    // Expose the WarpX instance
+    m.def("get_instance", &WarpX::GetInstance,
+        py::return_value_policy::reference,
+        "Return a reference to the WarpX object.");
+
     // Expose the python callback function installation and removal functions
     m.def("add_python_callback", &InstallPythonCallback);
     m.def("remove_python_callback", &ClearPythonCallback);