Merge develop into master

[peteschultz]

This pull request brings the master branch up to date with the develop branch.

• Updates for cuDNN v.8
• Manages target cuda architectures using mechanisms in CMake's FindCUDA package to manage cuda architectures
• MomentumConn reads, writes, and checkpoints the previous dW
• Frame numbers can be specified with a negative index, which indicates counting backwards from the end
• Adds a CMake option to control whether ctest calls "mpiexec -n 1" when running single-process tests
• Positions within probe output files are maintained in checkpointing
• Probes on non-root processes send output to root processes instead of writing data themselves
• Various bugfixes and code enhancements