Allow config variables to be used inside config variables

[ajwt]

We've a situation where we'd like to customise launch_parallel, and pass an extra argument to it in some cases.

The default is
launch_parallel = 'mpirun -np tc.nprocs'

but for various configs we'd like to have e.g.
launch_parallel='mpirun -np tc.nprocs --my-other-option'
or
launch_parallel='mpirun -np tc.nprocs --my-special-option'

Would it be possible to have a variable like
launch_parallel_template='mpirun -np tc.nprocs'
(which is a thing we might customise depending on the MPI implementation)

and then in the individual configs
launch_parallel='tc.launch_parallel_template --my-other-option'
or
launch_parallel='tc.launch_parallel_template --my-special-option'

[jsspencer]

Might be easier to extend

launch_parallel='mpirun -np tc.nprocs tc.args'

(similar to the run_cmd_template)

and then set tc.args in individual configs? It's been years since I looked at this code and threading this through might be quite annoying...