add basic coil objectives (#853)

Adds coil length, torsion, curvature objectives

Resolves #895 
Resolves #897 
Resolves #896 
Resolves #863

desc/backend.py

@@ -77,7 +77,14 @@
     from jax.experimental.ode import odeint
     from jax.scipy.linalg import block_diag, cho_factor, cho_solve, qr, solve_triangular
     from jax.scipy.special import gammaln, logsumexp
-    from jax.tree_util import register_pytree_node, tree_flatten, tree_unflatten
+    from jax.tree_util import (
+        register_pytree_node,
+        tree_flatten,
+        tree_leaves,
+        tree_map,
+        tree_structure,
+        tree_unflatten,
+    )
 
     def put(arr, inds, vals):
         """Functional interface for array "fancy indexing".
@@ -393,6 +400,18 @@ def tree_unflatten(*args, **kwargs):
         """Unflatten pytree for numpy backend."""
         raise NotImplementedError
 
+    def tree_map(*args, **kwargs):
+        """Map pytree for numpy backend."""
+        raise NotImplementedError
+
+    def tree_structure(*args, **kwargs):
+        """Get structure of pytree for numpy backend."""
+        raise NotImplementedError
+
+    def tree_leaves(*args, **kwargs):
+        """Get leaves of pytree for numpy backend."""
+        raise NotImplementedError
+
     def register_pytree_node(foo, *args):
         """Dummy decorator for non-jax pytrees."""
         return foo

desc/coils.py

@@ -133,7 +133,7 @@ class _Coil(_MagneticField, Optimizable, ABC):
     _io_attrs_ = _MagneticField._io_attrs_ + ["_current"]
 
     def __init__(self, current, *args, **kwargs):
-        self._current = float(current)
+        self._current = float(np.squeeze(current))
         super().__init__(*args, **kwargs)
 
     @optimizable_parameter
@@ -145,7 +145,7 @@ def current(self):
     @current.setter
     def current(self, new):
         assert jnp.isscalar(new) or new.size == 1
-        self._current = float(new)
+        self._current = float(np.squeeze(new))
 
     def compute_magnetic_field(
         self, coords, params=None, basis="rpz", source_grid=None

desc/objectives/__init__.py

@@ -1,6 +1,7 @@
 """Classes defining objectives for equilibrium and optimization."""
 
 from ._bootstrap import BootstrapRedlConsistency
+from ._coils import CoilCurvature, CoilLength, CoilTorsion
 from ._equilibrium import (
     CurrentDensity,
     Energy,