General: Add issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**Steps to reproduce**
+If possible, provide a *Minimal Reproducable Example*. Describe the steps to reproduce the bug.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Actual behavior**
+A clear and concise description of what actually happens.
+
+**System (please complete the following information):**
+ - OS: [e.g. Ubuntu 18.04, Windows 10]
+ - Compiler: [e.g. GCC 7, MSVC 19.2x (Visual Studio 2019)]
+ - Backend: [e.g. CUDA, OpenMP]
+ - Library version [e.g. 1.0.0, master]

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is.
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.