Added issue templates for GitLab under ".gitlab/" directory.

.gitlab/issue_templates/Bug.md

+<!---
+Please read this!
+
+Before opening a new issue, make sure to search for keywords in the issues
+filtered by the "Bug" or "Broken" label:
+
+https://gitlab.lrz.de/vadere/vadere/issues
+--->
+
+### Summary
+
+What is the problem you observed?
+
+### What is the current *bug* behavior?
+
+What actually happens?
+
+### What is the expected *correct* behavior?
+
+What you should see instead?
+
+### Steps to reproduce
+
+How can you reproduce the issue?
+
+### Relevant data
+
+- [If you report a bug] Git branch name and commit hash: branch/hash
+- [If you report a bug in a scenario] Please, provide the scenario file. No matter how simple the scenario is.
+
+### Logs and outputs
+
+[If relevant] Provide log files or console output with error/warning message.
+
+Please, use code blocks (backticks) and if it is much text encapsulate the text in `<details>` tag:
+
+<details>
+<summary>Expand file xy...</summary>
+<pre>
+```
+Line 1 of log file xy`
+Line 2 of log file xy`
+```
+</pre>
+</details>
+
+### Starting point
+
+[If you know it] Starting point in the code for handling the issue (e.g. class name) --> URL to code line in GitLab is also useful.

.gitlab/issue_templates/Feature.md

+### Problem to solve
+
+### Further details
+
+Include use cases, benefits, and/or goals.
+
+### Links and references
+
+- https://www.google.com/
+- http://www.vadere.org/