Implement Symbolic Recursive Integration for Dommaschk

CHANGELOG.md

@@ -14,6 +14,7 @@ New Features
 - ``desc.objectives.Omnigenity`` is now vectorized and able to optimize multiple surfaces at the same time. Previously it was required to use a different objective for each surface.
 - Adds a new objective ``desc.objectives.MirrorRatio`` for targeting a particular mirror ratio on each flux surface, for either an ``Equilibrium`` or ``OmnigenousField``.
 - Adds the output quantities ``wb`` and ``wp`` to ``VMECIO.save``.
+- Change implementation of Dommaschk potentials to use recursive algorithm and symbolic integration.
 - Changes hessian computation to use chunked ``jacfwd`` and ``jacrev``, allowing ``jac_chunk_size`` to now reduce hessian memory usage as well.
 
 Bug Fixes

desc/backend.py

@@ -5,6 +5,7 @@
 import warnings
 
 import numpy as np
+import sympy as sp  # noqa: F401
 from termcolor import colored
 
 import desc