Allow `CoilXXX` objectives also target curves, or add error if a curve is passed in

[dpanici]

It would be nice to allow our coil objectives (like CoilLength, CoilCurvature) also allow for optimization of curve objects, or if that is not easily doable, to at least throw an error if a curve is passed in (like errorif(not isinstance(things,Coil) or something), as one could naively assume (like I) that they could optimize geometry of curves as well.

this is a test that shows the issue

    c = FourierPlanarCurve()
    objective = ObjectiveFunction(CoilLength(c, target=11))
    optimizer = Optimizer("fmintr")
    (c,), _ = optimizer.optimize(c, objective=objective, maxiter=200, ftol=0, xtol=0)
    np.testing.assert_allclose(c.compute("length")["length"], 11, atol=1e-3)

has an error when the objective is built

desc/optimize/optimizer.py:217: in optimize
    objective.build(verbose=verbose)
desc/objectives/objective_funs.py:173: in build
    objective.build(use_jit=self.use_jit, verbose=verbose)
desc/objectives/_coils.py:337: in build
    super().build(use_jit=use_jit, verbose=verbose)
desc/objectives/_coils.py:153: in build
    self._grid = tree_map(
../../miniconda3/envs/desc-env/lib/python3.12/site-packages/jax/_src/tree_util.py:312: in tree_map
    return treedef.unflatten(f(*xs) for xs in zip(*all_leaves))
../../miniconda3/envs/desc-env/lib/python3.12/site-packages/jax/_src/tree_util.py:312: in <genexpr>
    return treedef.unflatten(f(*xs) for xs in zip(*all_leaves))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

x = array([0., 1., 0.])

    lambda x: LinearGrid(
>       N=2 * x.N + 5, NFP=getattr(x, "NFP", 1), endpoint=False
    ),
    self.things[0],
    is_leaf=lambda x: is_single_coil(x),
E   AttributeError: 'numpy.ndarray' object has no attribute 'N'

desc/objectives/_coils.py:155: AttributeError