The r.run_in_parallel script incorrectly checks the core count on head nodes when not on SLURM

[grantcurell]

I'm running in an environment without SLURM for testing and just realized that this math is run on my headnode despite a nodelist being passed via the PARALLEL_NODEFILE variable.

This causes MaxCores to be incorrectly calculated and later on in the code it erroneously says that there aren't enough cores available.

I'm running runmod directly from the command line.