Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve POT
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1 ...
+2. 
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Desktop (please complete the following information):**
+ - OS: [e.g. MacOSX, Windows, Ubuntu]
+ - Python version [2.7,3.6]
+- How was POT installed [source, pip, conda]
+
+Output of the following code snippet:
+```python
+import platform; print(platform.platform())
+import sys; print("Python", sys.version)
+import numpy; print("NumPy", numpy.__version__)
+import scipy; print("SciPy", scipy.__version__)
+import ot; print("POT", ot.__version__)
+```
+
+**Additional context**
+Add any other context about the problem here.