This commit adds an additional step when aggregating the values for
sending invoice CFDI, and ensures the final result to be within the
allowed maximum rounding error of `0.01`.

Purpose:

When creating an invoice with the following amounts:

- line 1: 505.0, tax_16
- line 2: 495.0, tax_16
- line 3: 475.0, tax_16
- set tax_16.price_include to True

The CFDI result of the invoice when generated using "round globally"
method on either normal or global sending will be rejected by the
mexican authorities.

This is caused by an inherent flaw of how Odoo calculates line amounts.
The `compute_all` method in taxes rounds all the lines in cfdi_values to
2 digits precision in the final result, resulting in a loss of precision
when calculating the total amounts.

The mexican authorities requires us to make sure that:

1. the sum of the base/tax amounts in the lines (Conceptos) matches the
   total
2. the rounding of the total base and tax amount to be within 0.01
   rounding error

In the provided example, in Odoo, we will generate a total of 1271.54
base amount and 203.46 tax amount. This is not acceptable by the mexican
government because 1271.54 * 0.16 equals 203.4464, which when compared
with our result of the tax amount exceeds the maximum allowed rounding
error.

To fix this, we have to sacrifice our correctness and generate amounts
that will have disrepancies whenever we need it (for global rounding
method).

We will now re-calculate the base and tax amounts in `cfdi_values` based
on the total amount and generate new amounts with 6 digits precision,
and apply that on each line of Conceptos.

The 2 digit rounding precision limit on `Importe` and `ValorUnitario` 
XML will be changed to 6 digits to make sure we pass the first
requirement, as the mexican authorities will also calculate their total
and check if it matches our total. Due to this change, all of the test
files than contain this element needs to be udpated too.


task-id: 4071712