Create pull_request_template.md

Author: joaomas

.github/pull_request_template.md

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+### 📝 Pull Request Summary
+
+_Briefly describe what this Pull Request adds or modifies in the MONAN model.<br>
+(...)_
+
+---
+
+### 🔧 Modification Type
+
+- [ ] Bug fix  
+- [ ] New feature  
+- [ ] Improvement to existing model functionality  
+- [ ] Code refactoring  
+- [ ] Other: _____________
+
+---
+
+### ✅ Testing and Code Quality
+
+- [ ] Code follows the MONAN DTN-01 coding standard  
+- [ ] The MONAN model was successfully compiled and executed  
+- [ ] New code is covered by unit or regression tests (if applicable)
+
+---
+
+### 🧪 Scientific Impact
+
+_If applicable, summarize the scientific or methodological relevance of this contribution.<br>
+(...)_