Create an input argument helper list

[victorapm]

The number of input arguments we accept via the YAML file, and their respective valid values, evolve over time. We need a simple way to tell users:

1. Which options they can use?
2. Which values are accepted by those options?

One idea is via command line, let's say:

$ ./hypre_app --help

  Usage: ./hypre_app input.yml

  List of valid sections for input.yml:

    general
    linear_system
    solver
    preconditioner

  For more information about which section: ./hypre_app --help <section>

Then, we might do:

$ ./hypre_app --help solver

  List of valid values for the solver:
     pcg - Preconditioned Conjugate Gradient
     gmres - Generalized Minimal RESidual
     fgmres - Flexible Generalized Minimal RESidual
     bicgstab - Bi-Conjugate Gradient STABilized

  List of valid keys under the solver section:

    max_iter: <val> - val is a positive integer number
    print_level: <val> - val is a positive integer number in {0, 1, 2}
    tol: <val> - val is a non-negative real number
    ...