Update Ιssue and PR templates (#434)

* Added Issue templates for bugs and new features
* Added PR template

Author: iomaganaris

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Describe the issue**
+A clear and concise description of what the issue is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+```bash
+A simple script
+```
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Logs**
+If possible attach helpful logs related to the issue.
+If there is an issue during build `CMakeError.log`, `CMakeOutput.log` or the output of `make VERBOSE=1` would be helpful.
+Otherwise any error printed to the therminal.
+
+**System (please complete the following information)**
+ - OS: [e.g. Ubuntu 20.04]
+ - Compiler: [e.g. PGI 20.9]
+ - Version: [e.g. master branch]
+ - Backend: [e.g. CPU]
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**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 about the feature request here.

.github/pull_request_template.md

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+**Description**
+
+Please include a summary of the change and which issue is fixed or which feature is added.
+
+- [ ] Issue 1 fixed
+- [ ] Issue 2 fixed
+- [ ] Feature 1 added
+- [ ] Feature 2 added
+
+Fixes # (issue)
+
+**How to test this?**
+
+Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce if there is no integration test added with this PR. Please also list any relevant details for your test configuration
+
+```bash
+cmake ..
+make -j8
+nrnivmodl mod
+./bin/nrnivmodl-core mod
+./x86_64/special script.py
+./x86_64/special-core --tstop=10 --datpath=coredat
+```
+
+**Test System**
+ - OS: [e.g. Ubuntu 20.04]
+ - Compiler: [e.g. PGI 20.9]
+ - Version: [e.g. master branch]
+ - Backend: [e.g. CPU]
+
+**Use certain branches for the SimulationStack CI**
+
+CI_BRANCHES:NEURON_BRANCH=master,