IfcSite does not reference the EPSG code for WGS84 and further inconsistencies with georef

Originally https://forums.buildingsmart.org/t/ifcsite-does-not-reference-the-epsg-code-for-wgs84-and-further-inconsistencies-with-georef/3499

The `IfcSite` documentation states WGS84, but since there are many WGS84s out there, we need to clarify which one.

> World Latitude at reference point (most likely defined in legal description). Defined as integer values for degrees, minutes, seconds, and, optionally, millionths of seconds with respect to the world geodetic system WGS84.
>
> World Longitude at reference point (most likely defined in legal description). Defined as integer values for degrees, minutes, seconds, and, optionally, millionths of seconds with respect to the world geodetic system WGS84.

I am not a domain expert, therefore I would assume it is EPSG:4326. However, I could be totally wrong :)

A further issue is that this RefLatitude and RefLongitude actually are based on the `IfcSite.ObjectPlacement`, whereas the `IfcMapConversion`, if it exists, is based on the origin. This inconsistency can only serve to create confusion and possible errors.

Also, is it irking anybody else that all this geolocation related stuff is spread around so many entities in the IFC spec? Can't it all be put in one spot? Can this be fixed for IFC5?