Branding: Rename to something less generic

[mattwthompson]

The current name, OpenFF System, is descriptive and roughly captures the intent of the project. It is, however, fairly generic and could get confused with other objects and keywords, i.e. OpenMM System, import os; os.system('cmd') in Python, etc.

This may no longer be necessary with the import paths (from openff.system import ...) resulting from our unified namespace.