Add default issue template

.gitlab/issue_templates/default.md

+## Summary
+
+(Summarize the bug encountered concisely)
+
+## Steps to reproduce
+
+(How one can reproduce the issue - this is very important! Please always upload `inputcard`, `potential` and `shapefun`)
+
+## What is the current bug behavior?
+
+(What actually happens)
+
+## What is the expected correct behavior?
+
+(What you should see instead)
+
+## Relevant logs and/or screenshots
+
+(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code, as
+it's very hard to read otherwise.)
+
+## Possible fixes
+
+(If you can, link to the line of code that might be responsible for the problem)
+
+/label ~bug ~reproduced ~needs-investigation
+