Remove dependency on 'nproc' for parallel tests

Requires compiling an extra tool, which while not really a burden has always felt a little awkward. I think it should be possible to check the number of threads by simply doing something like:
```sh
N_THREADS=`$DO_PARALLEL echo Hello Cpptraj | grep 'Hello Cpptraj' | wc -l`
```
Assuming `wc` is cross-platform.