Add AEP and PR template with ref to contrib guidelines and license (#1319)

.github/ISSUE_TEMPLATE/bug_report.md

@@ -3,27 +3,13 @@ name: Bug report
 about: Create a report to help us improve. If you need help setting up your simulation
   with Arbor, please start a new topic in Discussions.
 title: ''
-labels: ''
+labels: 'bug'
 assignees: ''
 
 ---
 
 **Describe the bug**
-A clear and concise description of what the bug is.
-
-**To Reproduce**
-Steps to reproduce the behavior:
-1. '...'
-
-**Expected behavior**
-A clear and concise description of what you expected to happen.
-
-**Screenshots**
-If applicable, add screenshots to help explain your problem.
-
-**System (please complete the following information):**
- - OS: [e.g. Ubuntu]
- - Version of relevant packages [e.g. Arbor 0.3, Conda Python 3.7]
+A clear and concise description of what the bug is, steps to reproduce, expected behavior, etc.
 
 **Additional context**
-Add any other context about the problem here.
+Add any other context about the problem here (e.g. OS, Arbor version, etc.).

.github/ISSUE_TEMPLATE/enhancement_proposal.md

+---
+name: Arbor Enhancement Proposal
+about: 'An outline and rationale for a new feature or major change to Arbor. '
+title: 'AEP: [PROPOSAL]'
+labels: 'AEP', 'enhancement'
+assignees: ''
+
+---

.github/ISSUE_TEMPLATE/feature_request.md

 ---
 name: Feature request
-about: 'Suggest an idea for this project. If you need help setting up your simulation
-  with Arbor, please start a new topic in Discussions. '
+about: 'Suggest an idea for Arbor. Add a clear and concise description of what the problem is and what you want to happen instead. '
 title: ''
-labels: ''
+labels: 'enhancement', 'help wanted'
 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 or screenshots about the feature request here.

.github/pull_request_template.md

+Please make sure your PR follows our [contribution guidelines](https://github.com/arbor-sim/arbor/tree/master/doc/contrib) and agree to the terms outlined in the [PR procedure](https://github.com/arbor-sim/arbor/tree/master/doc/contrib/pr.rst).