Add download callback to ClimaArtifactsHelper

[imreddyTeja]

There are two callbacks in this PR. The first only prints the download progress, while the other also prints the download rate.

For both, total size is printed as MB as an int if it is less than 1 GB, and GB with 2 decimal places otherwise. The same applies to dowloaded size and download rate.

The second callback is created with the function create_download_rate_callback which returns
a function. The callback calculates the
download rate over an interval, and prints the result at the end of
the interval. It also prints download progress. This callback
uses a closure to keep track of the time and bytes downloaded
at the end of the previous interval. If this callback is good,
then we can probably remove download_progress_callback from this PR.

I considered creating a wrapper for the download function, which would take in the file url and file name, and use the callback while downloading it, and then change the file name. This seems like it might be excessive. Thoughts?

Should I go through the existing artifacts and add the callback to the create_artifact.jl scripts?

Closes #68

[Sbozzolo]

Is it possible to create a Downloader that uses this in ClimaArtifactsHelper? So that people would import the downloader and use it directly