Maintain Higher-Order Derivative Representations

[pulsipher]

Currently, higher-order derivatives are built recursively as 1st-order ones by introducing auxiliary variables. This impedes the use of higher-fidelity solution approaches.

We should refactor the derivative modeling abstraction such that derivatives in InfiniteModels are stored with the order specified by the user. These can later be reformulated as needed by the transformation employed. Note that this does denote a breaking change to the underlying data structure, but the high-level modeling API will not be affected.