Add Constructions

Constructions.md

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+A construction is a shortcut in dyna3 for creating some entities and adding constraints so that the constructed object in relation to previously defined ones is produced. Here is a list of constructions we might want supported.
+
+- midpoint/centroid
+- perpendicular bisector
+- angle/dihedral angle bisector
+- plane through three points
+- line through two points
+- line segment defined by two points
+- polygon defined by n points
+- regular polygon
+- various stock polyhedra
+- cone with base and apex
+- prism with base and height
+- parallel to plane through point
+- sphere through four points
+- translate/reflection/rotation of an entity or entities
+
+Feel free to add more!