feat(centers && groups) : Centers and groups list (#238)

*feat(centers && groups) : Centers and groups list (#238)

src/app/centers/centers.service.ts

@@ -0,0 +1,17 @@
+import { Injectable } from '@angular/core';
+import { HttpClient, HttpParams } from '@angular/common/http';
+import { Observable } from 'rxjs';
+
+@Injectable({
+    providedIn: 'root'
+  })
+
+
+export class CentersService {
+    constructor(private http: HttpClient) { }
+
+    getCenters(): Observable<any> {
+        return  this.http.get('/centers');
+    }
+
+}