Move logic of which pyclaw to import (pyclaw vs petclaw) to run_app_from_main.

This PR moves the

    if use_petsc:
      import clawpack.petclaw as pyclaw
    else:
      from clawpack import pyclaw

logic that is current present in most (all?) examples into the 'run_app_from_main' utility.  This will allow new backends to be tested without repeating the above logic in every application.  For example, suppose a new backend called 'boxclaw' is created.  Currently one would have to change the above code snippet to:

    if use_petsc:
      import clawpack.petclaw as pyclaw
    elif use_boxlib:
      import clawpack.boxclaw as pyclaw
    else:
      from clawpack import pyclaw

in every example.

At this time I have only modified one example to demonstrate the intended usage.