Add Enhancement template

.gitlab/issue_templates/enhancement.md

+# Proposers
+
+- name-of-proposer-1 (institution-of-proposer-1)
+- name-of-proposer-2 (institution-of-proposer-2)
+...
+
+# Description
+
+Describe your proposal in ~1000 characters.
+You can reference external content listed in section "References" as [Ref-1].
+
+# Demo or definition of done
+
+Describe which high level conditions needs to be fulfilled to demonstrate this feature implementation is completed.
+You can reference external content (example, demo paper) listed in section "References" as [Ref-2].
+
+# References
+
+1. [Reference name](https://reference-url)
+2. Author1, Author2, Author3, et. al., “My demo using feature,” in Conference-Name Demo Track, 20XX.