Mast observations

Hello,

I came across some difficulty using the download_file method in astroquery/mast/observations.py when downloading just one file. I got a better result in my tasks when I added to lines 536:

# create a local file path if none is input. Use current directory as default.
filename = os.path.basename(uri)
if not local_path:
local_path = os.path.join(os.path.abspath ('.'), filename)
else:
local_path = os.path.join(os.path.abspath (local_path), filename)

I would like to suggest a modification of these lines in the code.

Best regards

Start in astroquery/mast/observations.py around line 536 and inspect how download_file handles a single file when local_path is omitted or supplied. Confirm the intended destination uses the downloaded URI's basename, then verify that single-file downloads land in the current directory or the requested directory without changing other behavior.

Written by the indexing model from the issue text.