CoilSet of SplineXYZCoil with different knots can give wrong values

[f0uriest]

I think there might be an issue with CoilSets if there are spline curves that have different breakpoints. We only vmap over params, not transforms, since its assumed that every coil in a CoilSet can use the same transforms. I think this is also an issue if they have different knots after #1099

Possible fixes:

• move knots and intervals back to params instead of transforms (not sure if this will work, what was the reason they were moved in the first place?)
• Add logic to desc.coils._check_type to ensure that knots and intervals are the same for all SplineXYZCoil in a CoilSet

Originally posted by @f0uriest in #970 (review)

Confirmed this is an issue on master:

coil = FourierPlanarCoil(1e6)
coil1 = coil.to_SplineXYZ(knots=2*np.pi*np.linspace(0,1,5))
coil2 = coil.to_SplineXYZ(knots=2*np.pi*np.linspace(0,1,5)**4)
coilset = CoilSet(coil1, coil2)
B = coilset.compute_magnetic_field(np.array([10, 1, 1]))
B12 = coil1.compute_magnetic_field(np.array([10, 1, 1])) + coil2.compute_magnetic_field(np.array([10, 1, 1]))
np.testing.assert_allclose(B,B12)

[dpanici]

@kianorr I vote the 2nd approach