Add methods to mphys interface for getting DV bounds and scaling factors

[A-CGray]

Adds methods to the TACSBuilder class to retrieve the DV bounds and scaling factors that have been defined in the element_callback. This makes it easy to define the DV bounds and scales when using TACS through MPhys, e.g

  dvs = self.add_subsystem("dvs", om.IndepVarComp(), promotes=["*"])
  structDVs = tacs_builder.get_initial_dvs()
  dvs.add_output("dv_struct", structDVs)
  lb, ub = tacs_builder.get_dv_bounds()
  structDVScaling = tacs_builder.get_dv_scalers()
  self.add_design_var(
      "dv_struct", lower=lb, upper=ub, scaler=structDVScaling
  )