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

@@ -42,7 +42,7 @@ build-py3:
         - rm -rf build workspace
         - mkdir build
         - cd build
-        - cmake -Wno-dev -C ../CMake/disable-trilinos.cmake ..
+        - cmake -Wno-dev ..
         - make -j 4
     artifacts:
         paths: