Unit updates and compatibility with UN/ECE codes

[samlown]

• Adds a few more "frequently used" unit keys for Mexico
• Brings support for being able to use either a unit key or UN/ECE unit code in the unit value (not entirely sure yet if this is genius or stupid.)

[cavalle]

Still not in love with making the unit field polymorphic:

• It feels hacky.
• It would be the only field in GOBL with two different formats/meanings.
• We are not guaranteeing it will be one of the valid UNECE codes (only that it has 2 or three uppercase letters)
• We can always extend the list of units for common UNECE units (which, IMO, should also include newton meters (UN) simply because it could become a common mistake for XUN)
• We could always use extensions if we really need them.

[samlown]

@cavalle I hear you, but I'm not convinced on the alternate paths at the moment. Also bare in mind that the previous version allowed any key in the unit field. You'll see in the commit there that some of the tests in Mexico were using an invalid unit, which wasn't great either.

While it feels a bit unusual, the JSON Schema is sound. Its just a string for which you can use a constant, or a code. The unit isn't used for decision making, which lessons the requirements, and I think its useful to be able to fall back on standard codes if the customer requires it.

I'll commit for the time being so we can fix some of the issues in Mexico.