Interact task: Contact keyframes

[nkhosh]

This PR introduces the task "Interact". Using key contact points with the environment, the user can guide the interaction with objects in the scene by adding the distance of contact point pairs as an objective term.

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[thowell]

Thanks for contributing this awesome task!

I will take a look at the code this week.

[nkhosh]

Thank you for your comments @thowell, I have applied the requested changes in my latest commit.

[thowell]

Thanks for making these changes! I sent you a PR that formats the C++ code. It can be merged into this PR. Otherwise, LGTM.

[thowell]

@nkhosh Thanks for making these changes! LGTM.