AWS S3 reads require key/authentication

[sandyfreelance]

Reading from the NASA TOPS's AWS cache for heliophysics, s3://gov-nasa-hdrl-data1/, works if user has an AWS account with keys, but not from a non-AWS user. AWS enables 'anonymous' access and the TOPS bucket is free and allows this. So minor change to cdfread.py needed for people not in a HelioCloud key environment (such as in other Jupyter hubs like 2i2c or via their laptop IDE).