Add options to customize execute_experiment.tpl

[scheibelp]

Closes https://github.com/LLNL/benchpark/pull/258

New variables supported in variables.yaml:

• extra_cmd_opts: These appear as extra options to mpirun, srun, or whatever exec cmd is chosen by scheduler. Newlines can be used for organization, but are removed ("-x 1\n-y 2" becomes mpirun -x 1 -y 2 ...)
• extra_batch_opts: This appear as batch directive lines at the top of the script
• post_exec_cmds: inserted verbatim after the exec (whatever the main experiment binary is run)
• pre_exec_cmds: like post_..., but before the experiment runs

This also provides a means to define a system or experiment-specific execute_experiment.tpl. The experiment can likewise choose to omit the allocation modifier, and thereby specify everything manually

[scheibelp]

@jdomke I'm curious if this PR addresses your needs

[jdomke]

@jdomke I'm curious if this PR addresses your needs

from a quick look at it, it seems to be doing what i needed, but i have not tried it, do you need me to verify it before you merge, or should i wait until after and debug if necessary?

[scheibelp]

do you need me to verify it before you merge, or should i wait until after and debug if necessary?

Whichever you prefer: it sounds like you are fine with it being merged before you get a chance to test it, so I'll proceed with that assumption unless you say otherwise.