IfcExternalSpatialElementTypeEnum

## Proposal

Type | Definition
---|---
EXTERNAL_FIRE | Space occupied by external fire safety zone (?)
EXTERNAL_ADJACENT | Space occupied by a neighboring ~~building~~ construction work
LAND_PARCEL | Space occupied by building plot or land parcel for example as documented by cadastral registries

Allow relationship of Pset_LandRegistration to IfcExternalSpatialElement.LAND_PARCEL

## Background

Pretty sure the description on EXTERNAL_FIRE is wrong:

> **EXTERNAL_FIRE**
>
> Space occupied by a neighboring ~~building~~ construction work.

We should probably add that though as EXTERNAL_ADJACENT (Specifically not naming it ADJACENT_BUILDING to be a bit domain agnostic).

I don't know what the definition of EXTERNAL_FIRE should be, is that some sort of safety zone?

There has been some form of consensus to not add geometry directly to IfcSite. ~In that line of thinking I'm inclined to say that this entity should be the designated entity to represent a building plot or lot~.