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/pytest_common.py

@@ -0,0 +1,12 @@
+import os
+from contextlib import contextmanager
+
+@contextmanager
+def pushd(path):
+    cwd = os.getcwd()
+    if not os.path.isdir(path):
+        os.makedirs(path)
+    os.chdir(path)
+    yield
+    os.chdir(cwd)
+