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/customer-discount.ts

@@ -1,12 +1,12 @@
 export class CustomerDiscount {
   id: string;
   name: string;
-  discount: number;
+  discount: number | null;
 
   public constructor(init?: Partial<CustomerDiscount>) {
     this.id = '';
     this.name = '';
-    this.discount = 0;
+    this.discount = null;
     Object.assign(this, init);
   }
 }