Add error message for non-supported scattering processes in DSMC (#6236)

Excitation and forward scattering is not yet supported in DSMC but
currently these processes do not result in sensible error messages.

Addresses #6229.

Signed-off-by: roelof-groenewald <[email protected]>

Author: roelof-groenewald

Source/Particles/Collision/BinaryCollision/DSMC/DSMCFunc.cpp

@@ -53,6 +53,10 @@ DSMCFunc::DSMCFunc (
 
         ScatteringProcess process(scattering_process, cross_section_file, energy);
 
+        WARPX_ALWAYS_ASSERT_WITH_MESSAGE(process.type() != ScatteringProcessType::EXCITATION,
+                                        "Excitation collisions are not yet supported in DSMC");
+        WARPX_ALWAYS_ASSERT_WITH_MESSAGE(process.type() != ScatteringProcessType::FORWARD,
+                                        "Forward scattering collisions are not yet supported in DSMC");
         WARPX_ALWAYS_ASSERT_WITH_MESSAGE(process.type() != ScatteringProcessType::INVALID,
                                         "Cannot add an unknown scattering process type");