Added a get_dt() method

Added a get_dt() to the opm.simulators.BlackOilSimulator Python module.
This will return the size of the previous simulator time step.

python/simulators/CMakeLists.txt

@@ -54,6 +54,11 @@ if(OPM_ENABLE_PYTHON_TESTS)
       COMMAND ${CMAKE_COMMAND}
       -E env PYTHONPATH=${PYTHON_PATH} ${PYTHON_EXECUTABLE}
       -m unittest test/test_schedule.py)
+  add_test(NAME python_throw
+      WORKING_DIRECTORY ${PROJECT_BINARY_DIR}/python
+      COMMAND ${CMAKE_COMMAND}
+      -E env PYTHONPATH=${PYTHON_PATH} ${PYTHON_EXECUTABLE}
+      -m unittest test/test_throw.py)
 endif()
 
 find_file(PYTHON_INSTALL_PY install.py