Solve memory leaks and add CI

• [bug] Solve cirular references induced by the usage of shared_ptr in Node
  • Node/Node
  • Node/GraphView
• [add] GitLab CI

.gitlab/ci/_global.gitlab-ci.yml

+################################################################################
+# Centralized definitions of common job parameter values.                      #
+# Parameters with many optional configurations may be in separate files.       #
+#                                                                              #
+################################################################################
+variables:
+  GIT_SUBMODULE_STRATEGY: recursive
+  OMP_NUM_THREADS: 4
+  GIT_SSL_NO_VERIFY: 1
+  DEBIAN_FRONTEND: noninteractive
+
+
+image: n2d2-ci/ubuntu20.04/cpu:latest
\ No newline at end of file