Add option to select order of pre and post coupling subsystems to aerostructural scenario.

[kejacobson]

The aerostructural _mphys_scenario_setup function was getting long with these changes, so I broke it up into more functions for each phase.

Also moved fake_ldxfer to unit_test subdir where it should have already been.

[anilyil]

The addition of the options are fine by me. I have one comment on the implementation though; I would add the check in one location in setup to make sure the provided list of disciplines are the correct ones. Then when we are actually adding the pre or postprocessing subsystems, we can just loop over the option instead of having if checks for each discipline since it looks like only the discipline name is changing.

I can make this change if you want @kejacobson. Let me know what you think.

[kejacobson]

The addition of the options are fine by me. I have one comment on the implementation though; I would add the check in one location in setup to make sure the provided list of disciplines are the correct ones. Then when we are actually adding the pre or postprocessing subsystems, we can just loop over the option instead of having if checks for each discipline since it looks like only the discipline name is changing.

I can make this change if you want @kejacobson. Let me know what you think.

I can add this.

[kejacobson]

I think we still need to check which discipline in the loop in order to know which builder to use. I also like keeping the else with a ValueError in there just to be safe.