Update the pr template
Add "Why is this needed".
I can get a description of what the PR does from the code changes,
answering the why question is more important.
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-### 1. What does this pull request do?
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+### 1. Why is this pull request needed and what does it do?
### 2. Which issues (if any) are related?
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### 3. Which documentation changes (if any) need to be made?
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