Added unit testing of triclinic boxes for communication

refs #2887
Use mapped buffer for particle migration in GPU communication

this was intended, but was somehow disabled
Fix compiler warning
updated a few commands to MPI_SUPPORTED

fixes #2792
Adapted plugins to double precision / Scalar

tested that both plugin templates compile

fixes #2541
Added explanation on GPU affinity to MPI documentation
Fixed bug with triclinic+MPI: cell list was not correctly padded

- this could have been a problem for strongly sheared boxes
- made sure unit test catches this
Remove debug print statement
Update a few things in compile and install docs
Clean up documentation warnings
fixed precision issue
Merge branch 'master' of https://codeblue.umich.edu/git/hoomd-blue