Integrate Python-based tests and benchmark into Travis

* Unfortunately, due to the SWIG dependency, we need sudo to install on
  Travis (conflicts when adding with debian-sid source prevent using addons)
  which means we cannot use the container-based infrastructure anymore.
* Building the Python bindings requires g++5 (at least with -Werr set).
* When building the Python bindings on Travis, the numpy includes are not found
  by cmake, so they have to be added separately by running a Python process with
  `numpy.get_include()`
* The build script now (somewhat clumsily) depends on the RUN_PYTEST environment
  variable. If it is set to anything other than "", it will make the tests and
  run tests and benchmarks

EKF/CMakeLists.txt

@@ -123,11 +123,15 @@ if(PythonTests)
   set_source_files_properties(../swig/ecl.i PROPERTIES CPLUSPLUS ON)
   set(swigged_sources ekf.h)
 
+  # Find numpy include
+  execute_process(COMMAND ${PYTHON_EXECUTABLE} -c "import numpy; print(numpy.get_include())" OUTPUT_VARIABLE NUMPY_INCLUDE_DIRS ERROR_QUIET)
+
   include_directories(
     ..
     ${MATRIX_DIR}
     ${EIGEN3_INCLUDE_DIR}
     ${PYTHON_INCLUDE_DIRS}
+    ${NUMPY_INCLUDE_DIRS}
     )
 
   set(SWIG_MODULE_ecl_EXTRA_DEPS ${swigged_sources})