Add support for NCAR's Derecho

[fergu]

This commit adds support for Maestro to use the PBS scheduler on NCAR's Derecho, via setting type: pbs_derecho under the batch block. In general, things should be broadly compatible with a generic PBS install, but since this is a Cray system there are a few potential oddities, which I have noted in the Docstring for the class.

As Derecho does support undersubscribing nodes via #PBS -l select=<nnodes>:ncpus=<ncpus>:mpiprocs=<mpiprocs>, this adapter accepts cores per task and reduces mpiprocs from procs per node to procs per node / cores per task, with a check if this would result in mpiprocs * cores per task > procs per node (by, e.g., specifying a cores per task that does not evenly divide procs per node.

For full disclosure, I did use Claude to help develop this, but I have read over the code it created, and have been using these edits myself.

[fergu]

I also did not add a test for this adapter. Looking at the tests directory, it looks like there's an assumption that the scheduler being tested is available in the test environment, and I wasn't sure if the tests should be able to run on Github CI or just on the machine of the person using it. I can add one for PBS if that's preferable.