Allow early termination of transform

[CosmoMatt]

Users may like to downsample a signal consistently, which is most straightforward in harmonic space. This may involve starting from a high resolution pixel-space map, computing harmonic coefficients and then inverse transforming at a lower resolution. If this is the goal, we may instead be able to terminate the recursive steps early and return a downsampled map with only the overhead of a forward and inverse pass at the lower resolution.

Another use case for this would be for convolutional layers in a UNet type network. These layers fall into the same class.