Replace xml.etree.ElementTree.fromstring functions with their defusedxml equivalents.

[lewismc]

DeepSource has flagged the following issues

In podaac/podaac.py

root = ET.fromstring(dataset.encode('utf-8'))

Justification is as follows

Using xml.etree.ElementTree.fromstring to parse untrusted XML data is known to be vulnerable to XML attacks. Replace xml.etree.ElementTree.fromstring with its defusedxml equivalent function or make sure defusedxml.defuse_stdlib() is called

[lewismc]

Full analysis can be see at https://deepsource.io/gh/nasa/podaacpy/issues/?type=security

[lewismc]

There are a few instances of this happening but it should be trivial to fix.

[lewismc]

Additional issues can be located at https://deepsource.io/gh/nasa/podaacpy/issues/