feat(aliby): method to pad dask array with zeros

WHY IS THIS CHANGE NEEDED?:
- to help create a dummy image instance with dimensions tiler likes

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- decided to separate these two steps: (1) add more dimensions to loaded
  image, (2) pad the dask array.  otherwise, i'd have to tell the
  pad_array function the existing dimensions -- it's easier to just
  assume it has five before running
- pad_array: just concat zero arrays with same dimensions as image_array

REFERENCES:
- issue #53