Add historic range query service

To discover the available historic ranges.

src/app/models/historic-range.model.ts

@@ -0,0 +1,4 @@
+export interface HistoricRange {
+    start_year: string;
+    end_year: string;
+}

src/app/services/historic-range.service.ts

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+import { Injectable } from '@angular/core';
+import { Observable } from 'rxjs/Rx';
+
+import { HistoricRange } from '../models/historic-range.model';
+import { ApiHttp } from '../auth/api-http.service';
+import { apiHost } from '../constants';
+
+/*
+ * Historic Range Service
+ * Returns available historic ranges from API
+ */
+@Injectable()
+export class HistoricRangeService {
+
+    constructor(private apiHttp: ApiHttp) {}
+
+    public list(): Observable<Scenario[]> {
+        const url = apiHost + '/api/historic-range/';
+        return this.apiHttp.get(url).map(resp => resp.json() || [] as HistoricRange[]);
+    }
+}