VentilationType Enum and VentilationControlMethods

[nllong]

see proposals/2019/Ventilation.md

[jasondegraw]

I'll approve this PR, but I am not a fan of the proliferation of "None" as a method to indicate the lack of something when not including the element is more effective.

[markborkum]

@jasondegraw In the context of BuildingSync enumerations, the terms "unknown/known" and "nothing/something" have distinct semantics when used in combination in assertions. There are three combinations:

1. We don't know.
2. We know that it is nothing.
3. We know that it is something.

In Haskell, this could be modeled by an algebraic data type:

data Assertion a = Unknown | Known (Maybe a)

where

data Maybe a = Nothing | Just a

is from the standard library.

The absence of an XML element has a different semantics to the above: "no assertion."

[jasondegraw]

@markborkum As I have previously pointed out, there are plenty of places where None has exactly no meaning in the schema. This exact context is not one of those cases, hence my approval.