Refactor Model and Node classes

[faisal-bhuiyan]

We need to refactor the Model class to store pointers to Node and (potentially) Constraint objects -- currently, we are holding copies to such objects which makes transformations difficult to keep track of.

Tasks

Change Data Structures:

• Modify the nodes vector to hold std::shared_ptr.
• Modify the constraints vector to hold std::shared_ptr.
• Add a std::vector<std::shared_ptr> to store beam elements.

Update Methods to Use Pointers:

• Refactor all methods that add nodes and constraints to return pointers instead of objects.
• Ensure proper handling and storage of these pointers in the nodes, constraints, and beam_elements vectors.

Node Transformations:

• Add methods to the Node class to perform transformations like rotation and translation.
• Ensure transformations are reflected in the model

Add Beam Elements:

• Implement a method to add BeamElement objects to the model.

Testing

• Create node, add to model, transform node, make assertion on node from model class

[faisal-bhuiyan]

This is almost close to completion -- will submit PR shortly for review.

[michaelasprague]

Under review; @faisal-bhuiyan addressing comments and should be ready for merge this afternoon.