Add CI with python 3 including trilinos

This Merge request adds a python3 build with the corresponding tests to the current CI pipeline.

A docker image including Trilinos has been created for this purpose.

Some remaining minor compatibility issues with python 3 have been fixed.

.gitlab-ci.yml

@@ -71,6 +71,7 @@ ctest-py2:
         - build-py2
 
 ctest-py3:
+    image: rboman/waves-py3
     stage: test
     script:
         - cd build