Added issue templates for Github. (epfl-ecps#3)

The templates will be used as a starting point whenever a new issue
is opened. Can be extended later to more issue types than just "Feature
request" and "Bug report" in case it's needed.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+
+---
+
+
+**Describe the bug**
+
+Summarize your issue here in a couple of sentences. We encourage
+you to try, as much as possible, to reduce your problem to the minimal example that still reproduces the issue. 
+That would help us a lot in fixing it quickly and effectively.
+Other than that, thanks for taking the time to contribute to Channelflow!
+
+**Expected Result**
+
+What you expected
+
+**Actual Result**
+
+What happened instead
+
+**Steps to reproduce the issue**
+
+```console
+$ mpirun simulate_flow ...
+<relevant output>
+...
+```
+
+**Information on your system**
+
+This includes:
+
+ 1. platform you are running on
+ 2. relevant configuration details (CMake configuration, etc.)
+
+**Additional context**
+
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+
+---
+
+**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.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.