Create PULL_REQUEST_TEMPLATE.md

Author: smithg09

.github/ISSUE_TEMPLATE/PULL_REQUEST_TEMPLATE.md

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+## Purpose
+_Describe the problem or feature in addition to a link to the issues._
+
+## Approach
+_How does this change address the problem?_
+
+#### Open Questions and Pre-Merge TODOs
+- [ ] Use github checklists. When solved, check the box and explain the answer.
+
+## Learning
+_Describe the research stage_