Adding indices at coords to other structured mesh types

[shimwell]

In this PR I added the method get_indices_at_coords to the structured meshes

#3824

we could add to the get_indices_at_coords RectilinearMesh and SphericalMesh mesh class

[EdenRochmanSharabi]

Hi, I'd like to take this on.

I've gone through PR #3824 discussion and the current state of the code. It looks like RectilinearMesh.get_indices_at_coords was already added (using np.searchsorted per axis with boundary clamping), so the only remaining piece is SphericalMesh.

My approach for SphericalMesh.get_indices_at_coords:

1. Accept Cartesian (x, y, z) coordinates (consistent with CylindricalMesh)
2. Convert to spherical (r, theta, phi) relative to self.origin:
   • r = sqrt(dx**2 + dy**2 + dz**2)
   • theta = arccos(dz / r) (polar angle from z-axis)
   • phi = atan2(dy, dx), normalized to [0, 2*pi]
3. Validate each value against its grid bounds, raising ValueError if outside (following CylindricalMesh precedent)
4. Find bin indices with np.searchsorted(..., side='right') - 1
5. Handle the degenerate case r = 0 where theta and phi are undefined: default to theta = 0, phi = 0
6. Return (r_index, theta_index, phi_index)

I'll also add tests following the existing pattern in test_mesh.py.

Does this approach sound right?