Add ability to animate shape variables to a gif in caps2tacs

[sean-engelstad]

• New routine tacs_model.animate_shape_variables can be used to help make gif files of a range of structural shape variables.
• No forward/adjoint analyses need to be run as that would slow down the animation process. Instead, a writeDummySensFile method is included since the tacsAIM postAnalysis expects function and derivative values before creating a new design.
• These features were successfully tested out on the simple_naca_wing.csm geometry in examples/caps_wing/7_animate_shape_variables.py (a new run script).
• This is useful in debugging the shape variable design spaces in caps2tacs.

[sean-engelstad]

• Just changed it so that only the current shape variable it is changing in the animation method is active, this way the mesh chain rule product is only computed for the current shape variable (much faster for multi-shape variable designs).