Add Python get_include() (#141)

Introduces a mscclpp.get_include() in the Python module.
The extension module is now named _mscclpp so that we can have
Python code in the mscclpp module.
Also does some miscellaneous cleanup.

python/core_py.cpp

@@ -143,7 +143,7 @@ void register_core(nb::module_& m) {
       .def("setup", &Communicator::setup);
 }
 
-NB_MODULE(mscclpp, m) {
+NB_MODULE(_mscclpp, m) {
   register_error(m);
   register_proxy_channel(m);
   register_sm_channel(m);