Refactor / enhance / document the plot functions

[kanekosh]

Type of issue

• New feature (non-breaking change which adds functionality)
• Documentation update

Description

Plot scripts plot_wing.py, plot_wingbox.py have a lot to be improved.

• Refactor: Two scripts have duplicated methods e.g. save_video. We should create a parent class to put the common parts.
• Refactor: API for plot should be unified for all cases; specify the type of case (multipoint or not, wingbox or not) as an argument to the plot function.
• We should be able to plot multipoint non-wingbox cases, as well as single-point wingbox ones.
• plot_wing should be callable in the python script.
• Update the document on visualization. In particular, add 1) how to run the plotter, 2) how you interpret the results (e.g. units of the lift).
• Convergence plot via pyOptSparse and OptView.