Integrate `pack_vector_fields` into `SphericalVector` Interpolation method.

[odlomax]

Hi @wdeconinck

This is part of issue #217. Here we've integrated the helper method from PR #218 into the SphericalVector interpolation method. Another CASE has been added to the interpolation test which demonstrates vector interpolation with components stored in individual fields.

Note: PR #223 needs to be merged before this.

@MarekWlasak, this is how I envisaged the use of this interpolation method within JEDI where we exclusively store vector field components in separate fields.

The final PR for issue #217 will integrate PR #220 into the helper and interpolation methods used here.