Improve the safety of file gatherer utilities

[Zeitsperre]

I have no idea if this is related, but I noticed that we download the MD5 file even if the netCDF file already exists:

xclim/xclim/testing/utils.py

Lines 189 to 196 in 939adcf

	if local_file.is_file():
	local_md5 = file_md5_checksum(local_file)
	try:
	url = "/".join((github_url, "raw", branch, md5_name.as_posix()))
	LOGGER.info(f"Attempting to fetch remote file md5: {md5_name.as_posix()}")
	urlretrieve(url, md5_file) # nosec
	with open(md5_file) as f:
	remote_md5 = f.read()

This means we are still writing to disk each time open_dataset is called. Maybe there some non-thread-safe things happenning? (I have no idea how it could resolve in a HDF error though).

We could try to:

• Download the MD5 to memory instead than to a file?
• Use locks to prevent multiple write attemps.
  ?

Originally posted by @aulemahal in #1258 (comment)

[Zeitsperre]

Relevant: xarray-contrib/cf-xarray#436