Feature: Tax Regimes are added so that different bills with different series can be printed for Different regimes such as VAT and GST

Chore: Model relationships updated to make them simpler
Chore: Bill printing majorly refactored for it

Due to the sheer depth of the changes. There can be showstoppers. Please test it carefully

bookie/src/app/regimes/regime-resolver.service.ts

@@ -0,0 +1,19 @@
+import { Injectable } from '@angular/core';
+import { ActivatedRouteSnapshot, Resolve } from '@angular/router';
+import { Observable } from 'rxjs';
+
+import { Regime } from '../core/regime';
+
+import { RegimeService } from './regime.service';
+
+@Injectable({
+  providedIn: 'root',
+})
+export class RegimeResolver implements Resolve<Regime> {
+  constructor(private ser: RegimeService) {}
+
+  resolve(route: ActivatedRouteSnapshot): Observable<Regime> {
+    const id = route.paramMap.get('id');
+    return this.ser.get(id ? +id : null);
+  }
+}