Mask.multiply should broadcast mask

It would be nice if ``Mask.multiply`` was able to broadcast itself for arrays of higher dimensionality. A common use case is if extracting a circular region from a spectral cube.

Start by locating the Mask.multiply implementation and checking how it handles arrays with more than two dimensions. Confirm the expected broadcasting behavior for extracting a circular region from a spectral cube, then verify that higher-dimensional inputs produce the intended masked result.

Written by the indexing model from the issue text.