The structure package defines the structural model.
A structure is defined as a Structure struct, which contains a map of nodes by id and a slice of elements.
Nodes are represented by the Node struct.
A node is identified by a unique id, has a position in the plane and an optional external constraint.
A Node can be created using one of the following functions:
MakeNode: requires an id, a position and external constraintMakeNodeAtPosition: requires an id, x and y coordinates and an external constraintMakeFreeNodeAtPosition: creates a non-constrained node with the given id and position.