Install only *.py files from nmodl directory

    - previously *.pyc and .so file from top level nmodl directory
      was getting copied to install prefix
    - this was causing issue mentioned in #139

Change-Id: I7f6bddbba71c48f9841d1f07e22f8479602ab0f3

src/pybind/CMakeLists.txt

@@ -47,4 +47,4 @@ add_custom_command(OUTPUT ${NMODL_PYTHON_FILES_OUT}
 # Install python binding components
 # =============================================================================
 install(TARGETS _nmodl DESTINATION lib/python/nmodl)
-install(DIRECTORY ${PROJECT_SOURCE_DIR}/nmodl DESTINATION lib/python/)
+install(DIRECTORY ${PROJECT_SOURCE_DIR}/nmodl DESTINATION lib/python/ FILES_MATCHING PATTERN "*.py")