First version with the doc conversion and the connection to the nav system.

I would appreciate any comments about the code structure, mainly how to handle the gateways and the different api calls. Right now, I have the nav.go which is the entry point but all the functions are basically the same as in the gateways package and I am just redirecting.

Th only thing missing about the doc conversion is when modifying an invoice (or a credit note) we need to include a reference for each line. This is not supported in GOBL, so I see 2 options:

• Add it as a field in gobl for each line and make the user include the line they are modifying
• Get the data from the invoice we are modifying and get the line number from the item name.