Update issue templates

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: Bug Report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Summary**
+
+(Summarize the bug encountered concisely)
+
+**Mem3DG version**
+
+(Paste the output of `pymem3dg.__version__` if you are using pymem3dg or value of `mem3dg::MEM3DG_VERSION` here.)
+
+**Steps to reproduce**
+
+(Please describe how we can reproduce the bug, and share all files needed. Submitting a minimal working example isolating the issue will improve response time.)
+
+**What is the current bug behavior?**
+
+(What actually happens)
+
+**What did you expect the correct behavior to be?**
+
+(What you should see instead)
+
+**Possible fixes**
+
+(Any suggestions or thoughts are welcome. If you can, link to the line of code that might be responsible for the problem)

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Summary**
+
+(Why would this be useful?)
+
+**Use cases**
+
+(Describe how the feature will make a difference - from a user point of view, not just developers.)
+
+**Impact**
+
+(What users would this matter to? Is it likely to have large impact relative to the amount of work required?)
+
+**Detailed description**
+
+(Describe the feature in detail. If this is e.g. a new algorithm you developed, are there a lot of people who have cited the paper, or other implementations available? How does it go beyond the current state of the code?)
+
+**Requirements**
+
+(Is there other stuff needed?)
+
+**Links/references/implementations**
+
+(Having a link to a paper can be a nice addition to a detailed description, but it's unlikely developers will have time to e.g. go and read a paper if you didn't have time to describe the feature in detail here. Would you be willing to write the implementation?)