Inconsistent with force field as 2 words

[bannanc]

The READMEs in this repository are still inconsistent about if force field is one word or two. The Mobley group has been using "force field" and I believe OpenFF agreed on this as well via Slack, but we should make sure we're consistent in our documentation and especially when we write our own name!

[j-wags]

Some guidelines:

• We should make sure the spacing is "Force field" anywhere in
  • user-facing text (markdown and reStructuredText),
  • jupyter notebook markdown blocks
  • python docstrings
  • mentions of the organization name ("Open Force Field")
• We should use the case-sensitive ForceField class name in direct references to the python class

This Issue can be closed when:

• A PR is merged where the git repo has been reviewed and instances of incorrect usage are fixed