GMSO 0.4.0

Features

• Allow creation of ForceField from etree.ElementTree (#440)
• Add group potentials in forcefield.py (#466)
• Add atomclass attribute to atomtype (#478)
• Add conversion to and from NetworkX (#472, #488, #491)
• Add conversion from Foyer XML (#438, #500)
• Make checking atom types optional in ForceField creation (#498)

Optimization

• Cached unit conversions when hashing unyt quantities (#469)

Maintenance

• Add python 3.8 support and remove channel mosdef (#490)