Futurize for python2/3

Context

This merge request brings compatibility for python 3 while ensuring backwards compatibility with python 2.

Details

The future package has been used to futurize the code. Waves can now depend on python3-xxx instead of python-xxx packages.

If python 2 is to be used instead of python 3, proceed with the usual build and add -DUSE_PY3=OFF to the cmake command line. Note that the additional python-futurize package is required.

Test

Passed on ubuntu18.04 (python 2.7.15 and 3.6.9), debian (gaston, python 2.7), msys2 (python 2.7.16 and 3.8.1)

CMake/FindMPI4PY.cmake

@@ -14,16 +14,16 @@
 
 if(NOT MPI4PY_INCLUDE_DIR)
     execute_process(COMMAND
-      "${PYTHON_EXECUTABLE}" "-c" "import mpi4py; print mpi4py.get_include()"
+      "${PYTHON_EXECUTABLE}" "-c" "from __future__ import print_function; import mpi4py; print(mpi4py.get_include())"
       OUTPUT_VARIABLE MPI4PY_INCLUDE_DIR
       RESULT_VARIABLE MPI4PY_COMMAND_RESULT
       OUTPUT_STRIP_TRAILING_WHITESPACE)
     if(MPI4PY_COMMAND_RESULT)
-        message("jfa: mpi4py not found")
+        message("waves/CMake/FindMPI4PY: mpi4py not found")
         set(MPI4PY_FOUND FALSE)
     else(MPI4PY_COMMAND_RESULT)
         if (MPI4PY_INCLUDE_DIR MATCHES "Traceback")
-            message("jfa: mpi4py matches traceback")
+            message("waves/CMake/FindMPI4PY: mpi4py matches traceback")
             ## Did not successfully include MPI4PY
             set(MPI4PY_FOUND FALSE)
         else (MPI4PY_INCLUDE_DIR MATCHES "Traceback")