Add get file function

Added a method to get images and spectra from the URLs supplied by the
SHA query function. Issues could occur if there are different character
encodings or file types, but utf-8 and fits have covered all the tested
cases so far.
Update test for get file

Commented out the actual get_img calls because they would create a
directoy and download files if run, potentially in parts of the file
system where the user/process does not have the right permissions.