Provide helpers for working with boolean slices

[Cadair]

NDCube for good reasons does not support slicing via a boolean array. However, a common pattern with Dataset is to generate a boolean array from logical operations on the headers table.
Therefore I think we should provide two helpers:

• files.slice_by_boolean_array which will let you slice to a single contiguous region of the Dataset based on a boolean slice.
• files.download_from_boolean_array which will only download FITS files where the array slice is true.
  (names tbd)

The slice_by_boolean_array could have options for how it changes the array into a contiguous slice, it could do first block, largest block containing all true elements etc.