Implement some standardization of dealing with design variables amongst solvers.

[kejacobson]

Although MPhys is designed to be modular, some of our runs scripts that swap multiple codes for the same discipline still end up with many if {this_solver} then {do this} blocks of code. One place where this tends to happen is dealing with design variables in independent variable components. If we can come up with a standard for this, that would simply our run scripts and user experience.

Here's a version of how I've starting generalizing this for FUN3D and our VLM solver. Both solvers implement a CaseDescription class that has a method to add its variables to an independent variable component, and a method to get a list of the variable names which can be used to generalize connecting the IVC variables to other components.

from attrs import define
import openmdao.api as om


@define(slots=False, kw_only=True)
class VlmSteadyCaseDescription:
    mach_number: float
    aoa: float = 0.0
    q_inf: float = 1.0
    q_inf_units: str = None
    velocity: float = 1.0
    nu: float = 0.0

    def add_variables_to_ivc(self, dvs: om.IndepVarComp):
        dvs.add_output("aoa", val=self.aoa, units="deg")
        dvs.add_output("mach", val=self.mach_number)
        dvs.add_output("nu", val=self.nu)
        dvs.add_output("vel", val=self.velocity)
        dvs.add_output("q_inf", val=self.q_inf, units=self.q_inf_units)

    def get_list_of_variable_names(self):
        return ["aoa", "mach", "nu", "vel", "q_inf"]

Example Usage:

        conditions = self.add_subsystem("conditions", om.IndepVarComp())
        aero_case_description.add_variables_to_ivc(conditions)
        
        # add MPhys scenario
        self.mphys_add_scenario(
            'scenario,
            ScenarioAeroStructural(
                struct_builder=struct_builder,
                aero_builder=aero_builder,
                ldxfer_builder=ldxfer_builder
            ),        

        # connect design variables to scenario
        for iv in aero_case_description.get_list_of_variable_names():
            self.connect(f"conditions.{iv}", f"scenario.{iv}")