KDTree example based on weebi Point of Sale anonymized dataset (#260)

* KDTree example based on weebi Point of Sale anonymized dataset

Author: flutter-painter

example/pos/README.md

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+# Point of Sale
+
+How to apply [ml_algo](https://pub.dev/packages/ml_algo) on point of sale data (i.e. tickets/invoices/bills) ?
+
+This first __KDTree__ example provides simple customer segmentation based on purchased items.
+Using it a business owner can identify customers who bought __similar items__.
+While identifying identical invoices is possible with standard _for_ loops,
+Only a KDTree algorithm provides enough flexibility to find __similar items__.
+
+```shell
+dart example/pos/kd_contacts.dart
+```
+
+## Walkthrough
+
+- The business owner selects a customer (e.g. contactId 35, aka Bobby)
+- The program finds Bobby's tickets and vectorize their items 
+- The program filters out Bobby's tickets and vectorize all the items
+- For each one of Bobby's tickets, the program find the nearest x3 and save them in a list 
+- The program selects the top 3 nearest customers based on bought items
+
+## Dataset
+
+- Origin : weebi.com (anonymized)
+- Location : West Africa
+- Timerange : March 2024 - March 2025
+
+- 59 articles
+- 52 contacts (customers)
+- 341 tickets