Fix ticket:413 by adding section to mesh1D example

Add a section to the `mesh1D` diffusion example showing the difference
between spatial parameter variation in heat transfer as compared to
mass transfer. Also, add relevant figures to the `generated` folder.
Add parenthesis to transient term in heat equation

Address #413
Merge branch 'ticket413-Remind_users_of_different_types_of_conservation_equations' into develop

Fix #413