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/core/regime.ts

@@ -0,0 +1,16 @@
+export class Regime {
+  id: number | undefined;
+  name: string;
+  header: string;
+  prefix: string;
+  isFixture: boolean;
+
+  public constructor(init?: Partial<Regime>) {
+    this.id = undefined;
+    this.name = '';
+    this.header = '';
+    this.prefix = '';
+    this.isFixture = false;
+    Object.assign(this, init);
+  }
+}