Notification system in frontend

- Heatmap worker now reports progress
- Base for new altitude value
- Phones can be marked active

backend/models/beat/beat.interface.ts

@@ -2,6 +2,7 @@ import { Document } from 'mongoose';
 import { IPhone } from '../phone/phone.interface';
 
 export interface IBeat extends Document {
+    // [latitude, longitude, altitude, accuracy, speed]
     coordinate?: number[],
     accuracy: number,
     speed: number,