Run CI tests on ubuntu 20.04

[paulromano]

This PR updates GitHub Actions to use Ubuntu 20.04 for our CI tests. Currently, we are using Ubuntu 16.04 which is officially EOL this month, after which it's not clear how long it's going to supported on CI platforms.

In order to get tests working on Ubuntu 16.04, I had to switch our MPI implementation from OpenMPI to MPICH. We actually just switched to OpenMPI in #1820, but it looks like it's not going to work on Ubuntu 20.04. The problem is that recent versions of OpenMPI do not allow recursive invocations of MPI_Init, and this actually happens in our test suite from the following logic:

import subprocess
from mpi4py import MPI

subprocess.run(['mpiexec', '-n', ...])

Importing mpi4py actually initializes MPI in the current process, and then calling a subprocess with mpiexec does it again. MPICH thankfully handles this fine.

There's also a few fixes in here for warnings that are raised when the test suite is run.

[pshriwise]

No comments from me. Thanks for heading off this issue, @paulromano!