Add interface for open_data_file()
Import open_data_file

This fixes compiler warnings.
Interface the rest of the subroutines
Add interface for limiter
Import subroutines

This fixes several warnings
Rework the forall loop to avoid a warning:

/home/ondrej/repos/classic/src/1d/hyperbolic_step.f90:113.16:

                f(m,i) = f(m,i) + 0.5d0 * abs(s(mw,i))      &
                1
Warning: The FORALL with index 'mw' is not used on the left side of the assignment at (1) and so might cause multiple assignment to this object
Merge pull request #16 from certik/interface

I have a fix for the Makefiles (at least in the `tests` directory).  Everything looks good on my end.