Update to external subsystem API

[jkirk5]

Hello Aviary users!

We have recently made a major API change to Aviary regarding how external subsystems are added. Previously, external subsystem builders needed to be provided in phase_info for each individual phase, including pre- and post-mission. The dev team disliked this for several reasons, primarily the tedious duplication for the simplest use case, using the subsystem everywhere, and the fact that it over-complicates phase_info with required python scripting (we want to be treating it as an input file).

The new way to add external subsystems is using the L1 or L2 API in your main run script. Below is an excerpt of code from run_custom_mass_example.py:

# create Aviary problem, load inputs like normal
prob = av.AviaryProblem()

prob.load_inputs(
    'models/aircraft/advanced_single_aisle/advanced_single_aisle_FLOPS.csv', phase_info
)

# add external subsystem using new method - pass as a list containing all builders you wish to add
prob.load_external_subsystems([WingMassBuilder()])

prob.check_and_preprocess_inputs()

# ... the rest of the problem is setup and run as usual

This adds your subsystem builder(s) to the AviaryProblem, where it will be automatically added everywhere your builder provides components for (pre-mission, mission, post-mission).

If you still need to be able to selectively add your subsystem per-phase, then you need to update it with a flag that can be passed via phase_info's subsystem_options for that phase, and have the subsystem not provide the mission components if the flag requests it. This is the new recommended method for dynamic, per-phase subsystem behavior.