Update issue templates

Added the Data Structure request template.

Author: aalhour

.github/ISSUE_TEMPLATE/data-structure-request.md

@@ -0,0 +1,26 @@
+---
+name: Data structure request
+about: Suggest a new data strcuture for this project
+title: 'Data structure request: [NAME]'
+labels: data structure request
+assignees: ''
+
+---
+
+**Is your data structure request related to a computational problem? Please describe.**
+A clear and concise description of what the problem is. For example, I am working on a new dictionary implementation and I'd like the internal functionality to be based on a BTree implementation.
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen. For example:
+
+> I'd like to have a generic BTree implementation where I can specify the types of the keys and values, so that I can define a higher-level interface for a dictionary based on that.
+
+Or, more concretely:
+
+> I'd like to have an implementation of the BTree Data Structure, based on the following research paper: [Organization and Maintenance of Large Ordered Indexes, Bayer & McCreight](https://infolab.usc.edu/csci585/Spring2010/den_ar/indexing.pdf).
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.