[Feature] Add a helper function to get the memory size of one image plane

[dstansby]

When I document the cell detection steps, I'm going to include a discussion of the memory use by each of the steps in the pipeline. Because the cell detection works on a number of planes, this discussion will probably be in terms of the size of a single plane of input data.

It would therefore be good to have a user-facing helper function that looks like:

def get_plane_size_in_memory(arr: Union[np.ndarray, da.Array]) -> float
    ...

Users could use this to quickly query how big their data is, and thus estimate how much memory they need to run the cell detection step.