use LaTeX to clean up plot labels

[mdb15106]

Matplotlib lets you use LaTeX notation to clean up plot labels. For example, instead of plt.xlabel('mf/m0'), you could use plt.xlabel('$m_f/m_0$') to have subscripts appear, or even plt.xlabel('$\dfrac{m_f}{m_0}$') to have the label appear formatted as a fraction.