Update HOOMD examples to HOOMD v3

[bdice]

The HOOMD examples should be updated to use HOOMD version 3 (currently in beta). Related to #14.

[bdice]

Update: this has come up a few times in conversations. We're going to wait to rewrite the tutorial content using HOOMD until version 3 is finalized. That will likely be a few months from the time of writing (February 2021).

[atravitz]

I'd like to keep HOOMD v2 examples alongside HOOMD v3 for at least a little while after the v3 examples is introduced.

[bdice]

I think that's fine -- we can just copy and rename the folder to something like flow.hoomd2.lj and flow.hoomd3.lj. We'll only be able to test one configuration on our CI, however. We would add a file .skipci in the project folder to disable CI on one or the other. I'd prefer to skip CI on HOOMD 2.

[bdice]

HOOMD v3 has been released. This is ready for work now.