feat(Jacobian): Jacobian generation is now stateless.

Previously Jacobians were stored by engines and accessed through engine
accessors (e.g getJacobianMatrixEntry); however, this resulted in
desynced jacobian states. We have changed to a pattern of Engine creates
a jacobian and returns it to the caller. The caller can then do what
they will with it. Because of this the getJacobianMatrixEntry method has
been removed.

BREAKING CHANGE:
    - There is no longer any getJacobianMatrixEntry method on
DynamicEngine classes
    - the generateJacobian method signature has changed to return a
NetworkJacobian object. Internally this uses an Eigen Sparse Matrix to
store its data.

src/include/gridfire/exceptions/error_solver.h

@@ -18,4 +18,8 @@ namespace gridfire::exceptions {
     class CVODESolverFailureError final : public SolverError {
         using SolverError::SolverError;
     };
+
+    class SingularJacobianError final : public SolverError {
+        using SolverError::SolverError;
+    };
 }