Enable modification of single datasets stored in an h5cube

[bskinn]

E.g., selectively thresholding only one field stored in the file.

This might only really be used to try to further compress a file -- it would require copying all of the data to a new .h5cube file, since I'm pretty sure shrinking a dataset within an existing file doesn't de-allocate any space within the file. Wouldn't be that hard to do, though.

Would want to implement both a py API and a command line interface for it.