Add design variable support for aux elements

[timryanb]

Summary
This PR enables entries of the inertia (acceleration) vector passed to addInertialLoad and pressures passed to addPressureToElements and addPressureLoadsToComponents and tractions passed to addTractionsToElements and addTractionsToComponents to be treated as design variables in an optimization. This allows, for example, the magnitude of a gravity/pressure load to be a DV without modifying the structural stiffness model.

Example

# Register gravity magnitude as a global DV (returns its DV number)
gravDVNum = FEAAssembler.addGlobalDV("g", -9.81)

# FEAAssembler intialization stuff
...

# Create problem
problem = FEAAssembler.createStaticProblem("dummy")

# z-component is the DV; x and y are fixed
g = np.array([0.0, 0.0, -9.81])
problem.addInertialLoad(g, [-1, -1, gravDVNum])