Run Python / ERT based integration tests.

Added a new optional dependency the ERT Python distribution to
opm-simulators, and a cmake function 'opm_add_python_test' which can be
used to simplify integration tests of the type:

 1. Run flow on a model.
 2. Compare the results with a well known reference solution.

cmake/Modules/OpmPythonTest.cmake

@@ -0,0 +1,7 @@
+function (opm_add_python_test TEST_NAME TEST_SCRIPT)
+    add_test(NAME ${TEST_NAME} 
+             WORKING_DIRECTORY ${PROJECT_BINARY_DIR}
+             COMMAND ${TEST_SCRIPT} ${ARGN})
+
+    set_property(TEST ${TEST_NAME} PROPERTY ENVIRONMENT "PYTHONPATH=${ERT_PYTHON_PATH}:${PYTHONPATH}")
+endfunction(opm_add_python_test)

cmake/Modules/opm-simulators-prereqs.cmake

@@ -19,7 +19,8 @@ set (opm-simulators_DEPS
 	# DUNE prerequisites
 	"dune-common REQUIRED;
 	dune-istl REQUIRED"
-	# OPM dependency
+	"ERTPython"
+        # OPM dependency
 	"opm-common REQUIRED;
         opm-parser REQUIRED;
 	opm-core REQUIRED;