This commit fixes the following faulty behaviors:

1. There is a traceback in the pos ui when the selected
customer has fiscal position that maps a tax to nothing.
To prevent this behavior, we avoid to add the undefined
tax destination.

2. When you change the customer in pos ui from someone who
has fiscal position to someone without, the tax in the
order remains to be based on the previous customer. The
tax should change to original tax of the product or
to the tax mapped by the default fiscal position of
the session. To avoid this behavior, we now set the
default fiscal position on the order when a customer has
no fiscal position.


**To reproduce:**

**Issue 1. Error when `product1` is selected for `partner1`**

*Steps:*
1. create fiscal position `fpos` that maps `tax1` to nothing
2. assign `fpos` to `partner1`
3. activate fiscal position per order in pos.config and add `fpos`
4. assign `tax1` to `product1` (should be available in pos)
5. open new pos session
6. select `product1`
7. issue: error when selecting `partner1` as customer

*Reason.* In `_map_tax_fiscal_position`, `tax=undefined` is pushed to `taxes`.

**Issue 2. Tax for `partner1` is active in `partner2` even if
`partner2` has no fiscal position.**

*Steps:*
1. create fiscal position `fpos` that maps `tax1` to `tax2`
2. assign `fpos` to `partner1`
3. activate fiscal position per order in pos.config and add `fpos`
4. assign `tax1` to `product1`
5. open new pos session
6. select `product1` -> tax should be `tax1` since no customer
7. select `partner1` -> tax changed to `tax2` because of `fpos`
in `partner1`
8. issue: select new customer without fiscal position -> tax
should be `product1`'s original tax because no fiscal position,
but tax from previous customer is not changed.

*Reason.* If the newly selected customer has no fiscal position,
order.fiscal_position is not updated so it takes the old fiscal position.

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Enforce the domain of the UOM not by an onchange but by a domain.

Example of an issue if the domain is enforced by an onchange:
- create an order
- add an order line
- select the product
- select an uom
> the uom presented are the one from the product category.
-Save
- edit
- select an uom
> all UOM are presented even the ones of other categories

This commit enforce this new logic at most places.

Description of the issue/feature this PR addresses:
Display only category's uom

Current behavior before PR:
At the time of editing a sale order line, it displays all category's uom.

Desired behavior after PR is merged:
At the time of creating or editing a sale order line, it displays only related category's uom.

Task: 
https://www.odoo.com/web#id=2003959&action=333&active_id=809&model=project.task&view_type=form&menu_id=4720

Pad: 
https://pad.odoo.com/p/r.35c17fb973d768886a273c818f42ea02

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