Add sub command that generates and submits the PBS job

[SeanBryan51]

We should add a sub command (fluxnet-submit-job) that writes the PBS job to the file system and then submits the job to the queue. This allows us to run the full fluxsite test by executing each step separately, i.e. benchcab checkout && benchcab build && benchcab fluxnet-setup-work-dir && benchcab fluxnet-submit-job.

This sub command should include the --skip option so that the bitwise comparison step can be disabled.

[SeanBryan51]

This is a good opportunity to update the help message for benchcab fluxnet:

Runs the default fluxnet test suite for CABLE. This command is the equivalent of running 'benchcab checkout',
-'benchcab build', 'benchcab fluxnet-setup-work-dir', and 'benchcab fluxnet-run-tasks' sequentially.
+'benchcab build', 'benchcab fluxnet-setup-work-dir', and 'benchcab fluxnet-submit-job' sequentially.