Merge branch 'ci_cd' into 'master'

add continuous integration support

See merge request zavolan_group/pipelines/rnaseqpipeline!5

.gitlab-ci.yml

+image: snakemake/snakemake:v5.9.1
+
+test:
+  script:
+    - cd snakemake
+    - snakemake -n
+    # add additional tests here
+