Structure_Engine: Add engine methods for retaining wall classes

[KalleEdstroem]

NOTE: Depends on

https://github.com/BHoM/BHoM/pull/1621

Issues addressed by this PR

Closes BHoM_Structure_oM

Test files

See. https://github.com/BHoM/BHoM/pull/1621

Changelog

Added required extension methods for implemented interfaces for new retaining wall classes.

• Added Geometry() method for Stem, Footing and RetainingWall
• Added IsNull() method for Stem, Footing and RetainingWall
• Added SolidVolume() method for Stem, Footing and RetainingWall
• Added MaterialComposition() method for Stem, Footing and RetainingWall
• Added SetOutlineElements(List<IElement1D> outlineElements1D) method for Stem and Footing
• Added OutlineElemets1D() method for Stem and Footing
• Added Geometry3D() method for Stem, Footing and RetainingWall

Additional comments

HasMargeablePropertiesWith(IElement2D) has not been added for any of the objects in reference to: https://github.com/BHoM/BHoM_Engine/pull/3138#discussion_r1312778359