Feature FEL POS

[Tropicalrambler]

Clone ERPNext's POS functionality, to create and Validate Point of Sales Invoices, allowing the generation of Electronic Invoices upon pressing "Submit".

Expected behavior:
User assembles an order in POS
User clicks on "Pay"

• User may enter multiple cards for each payment method (credit or debit), with corresponding amount.

User indicates payment method, and in case of credit card,

• allows entry of a payment confirmation Number for each card.
  User clicks on Submit

• Add triggers to calculate POS based items sales tax and excise tax

• Allow calculation of excise taxes using Python for automated generation

• ERPNext submits invoice, then proceed to generate electronic invoice. If failed, show generation error, invoice must be left in submitted state, and a new cart open. If successful, continue to print screen, showing the print format for POS. Sales Invoice FEL numbers must be included.