Everything ifc

[ikeough]

BACKGROUND:
Previously we could only import elements like Walls, Columns, and Beams from IFC. This is because we mapped IFC entities to our corresponding core types. As a short term fix (knowing we're also going to move core types out of elements), we can drop the exact mapping and import everything as a geometric element. This will (I think) enable us to use imported IFC geometry as snap targets.

DESCRIPTION:

• This PR imports everything as a GeometricElement.
• This PR adds Openings to GeometricElement to align with IfcBuildingElement.
• After adding Openings to the GeometricElement the IHasOpenings interface is no longer required and it is removed.

REQUIRED:

• All changes are up to date in CHANGELOG.md.

COMMENTS:

• Any other notes.

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This change is 

[ikeough]

One small piece I still need to do. I added a method FromIFC2 to get around the existing FromIFC. If we agree with the direction of this work, the original method will be removed and replaced with the new one.

[ikeough]

After conversation with @andrewheumann, the following changes are proposed.

• Add a new base type BuildingElement, from which Things like Wall and Beam can inherit. Add the Openings property to this type.

Future Work:

• Move building elements to a separate library Elements.BuildingElements, which contributes to keeping the core small.
• Add the BuildingElement.IfcEntityType property to allow a user to define which type of IFC Entity they would like to map to during export. This will allow any building element type to map to any IFC entity type, but will also run the risk of creating illegal IFC entities.

[ikeough]

This is waiting on work to support IndexedPolycurve.

[DmytroMuravskyi]

What is missing in this project?