0.2 docs update

[j-wags]

This PR does a lot of housekeeping, including:

• Versions the toolkit as 0.2.0
• Versions the SMIRNOFF spec as 0.2
• Puts unstable API warnings on most classes in the toolkit
• Polishes up many areas of the docs
• Replaces hard links to /topology urls with ones to latest and master
• Fixing the non-loading images on the SMIRNOFF spec page
• Removes the "open questions" text on the SMIRNOFF spec page (added it to issue Address open questions on SMIRNOFF format spec revamp standards#48 )
• Adds LGTM and CodeCov badges
• Fixes Move PEP8 API warning into ChemicalEnvironments docstring #221

New docs can be found here: https://open-forcefield-toolkit.readthedocs.io/en/0-2-docs-update/

[j-wags]

This pull request fixes 1 alert when merging eaaf7db into 32273d5 - view on LGTM.com

fixed alerts:

• 1 for Testing equality to None

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Comment posted by LGTM.com

[j-wags]

@jchodera -- Remaining questions not addressed in comments above:

Should we rename Chemical-starting-points.md to FAQ.md to more accurately reflect its title and contents?

Good idea. I went ahead and did it.

Eventually, we may want the SMIRNOFF spec to live in its own separate GitHub repo.

I'm ambivalent, but haven't thought too hard about this. Added to openforcefield/standards#48. We can make this its own issue if it builds traction.

[j-wags]

This pull request fixes 1 alert when merging 3f184a1 into 32273d5 - view on LGTM.com

fixed alerts:

• 1 for Testing equality to None

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Comment posted by LGTM.com

[jchodera]

Looks like you're good to go!

[j-wags]

This pull request fixes 1 alert when merging e2d76b4 into 32273d5 - view on LGTM.com

fixed alerts:

• 1 for Testing equality to None

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Comment posted by LGTM.com