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/test/test_schedule.py

@@ -9,16 +9,7 @@ from opm.io.parser import Parser
 from opm.io.ecl_state import EclipseState
 from opm.io.schedule import Schedule
 from opm.io.summary import SummaryConfig
-
-@contextmanager
-def pushd(path):
-    cwd = os.getcwd()
-    if not os.path.isdir(path):
-        os.makedirs(path)
-    os.chdir(path)
-    yield
-    os.chdir(cwd)
-
+from .pytest_common import pushd
 
 class TestBasic(unittest.TestCase):
     @classmethod