Fix use_cluster

If either the command or the environment disable cluster usage, run
locally.
Add a method to set partition
Set ncores via environment variable

Prevents targets from rebuilding if the number of processors changes.
Add timelimit for jobs submitted with srun
Add timelimit for jobs submitted with salloc

Also modify test script to show SLURM job information
Default to one core with srun

Using SRun over Command with a single core can be useful when specifying
a time limit.
Set partition in initialization
Have command delegate to SRun
Add a few flags to speed up builds
Update docs for SLURM use
add option to print one extraneous file per line
whitespace
Don't create Refpkgs

And some pyflakes motivated cleanup