Enhance input arguments setup

[victorapm]

Allow users to oversubscribe input arguments passed from the YAML file via command line.

Example:

Suppose we have a YAML input like:

solver: gmres
  max_iter: 50
  print_level: 1
  tol: 1.0e-9

The idea is to allow users to oversubscribe, let's say the max_iter parameter, via command line such as:

$ ./hypre_app input.yml --solver:gmres:max_iter 100

Note the YAML (key, val) pairs are effectively being concatenated with the : character.

Also, we would like this to work:

  $ ./hypre_app input.yml --linear_system:execution_policy host
  $ ./hypre_app input.yml --linear_system:execution_policy device