Added issue templates for bug reports, feature requests, and questions

Author: merendamattia

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Reporting erroneous/inconsistent behavior
+title: "[BUG] "
+labels: "bug"
+assignees: 
+  - merendamattia
+  - VincenzoArceri
+
+---
+
+**Description**
+A clear and concise description of what the bug is.
+
+**Reproducibility information**
+Report the version (or the commit hash) to identify the point in time where this bug happens. Try to refer to commits already present on the master branch if possible.
+
+**Expected behavior**
+Describe what is expected to happen (output, logs, exceptions, ...).
+
+**Actual behavior**
+Describe what is happening instead (output, logs, exceptions, ...).
+
+**Attachments**
+If applicable, attach everything that might help us reproduce the bug.
+
+**Further content**
+Add anything else that might be important for resolving this bug.

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: "[FEATURE REQUEST] "
+labels: 'enhancement'
+assignees:
+  - merendamattia
+  - VincenzoArceri
+
+---
+
+**Description**
+Describe what the requested feature is.
+
+**Motivation**
+Describe why you think this feature is useful.
+
+**Suggested implementation**
+If you are aware of a paper presenting the required feature (if any), please cite it here.
+
+**Further content**
+Add anything else that might be important for implementing this feature.

.github/ISSUE_TEMPLATE/question.md

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+---
+name: Question
+about: A question for the developers
+title: "[QUESTION] "
+labels: "question"
+assignees:
+  - merendamattia
+  - VincenzoArceri
+
+---
+
+**Description**
+Write your question here!