Run automated / CI testing

[mphoward]

Now that azplugins is available publicly on github, I need to find a new testing / CI platform since the old Jenkins server is no longer available. CircleCI seems like a good option at least for the CPU tests. (There aren't any GPUs available for execution.) To get those tests up and running, I need:

• [x] Build docker containers based on HOOMD's ci images.

These containers will have a few different build configurations of hoomd installed, and then the plugins can be built externally to those to save on compile time. We probably need two versions of hoomd: (1) the minimum supported version and (2) "bleeding edge" hoomd (either the latest tag or maint) to look out for bugs. (2) might be better suited to a download-and-build since this code may continually change.

• [x] Setup a CircleCI config file to build and run the various test configurations on the CPU.
• [ ] Add testing for the documentation once it has been built (#1)

A long-term testing option is needed for the GPU if significant code development is going to continue on this project.

[mphoward]

Somewhere, we need to test a mix of CUDA versions, as some compilation errors only show up in CUDA 9 (see #4).

[mphoward]

Will defer doc testing until #1 is completed/

[mphoward]

Implemented on master and mostly polished off in #7 .