Numerics config options

[jimc101]

The NUMERICS section of the configuration file could be confusing, especially to new users. It could be made clearer by adding separate sections for the different numerical methods (standard, operator_split_1/2). Within these sections, there could be separate options for the numerical integration scheme used for advection and diffusion, and their respective time steps. This could impact attempts to return the global time step, and an alternative method for computing this may be required. Adding a method to num method objects that allows this to be returned could work.