Spherical harmonics

We can compute spherical harmonics directly from the nodal points on the cubed sphere mesh. The easiest way is probably to compute the dot product with the different basis functions.

```
using AssociatedLegendrePolynomials
λlm(l, m, sind(lat)) * complex(cosd(m * long), sind(m * long))
```

Questions:
- how high `l` and `m` are required?
- do we need to use higher-order quadrature to compute the dot products?
- how should vector fields be handled?
- just compute the scalar spherical harmonics of u and v? this could cause issues at the pole?
- use the Swarztrauber (1993) approach?