Consider moving cubed_a2d subroutine to fv_grid_utils.F90

[spencerkclark]

As part of #34, cubed_a2d from external_ic.F90, needed for restarting the model using A-grid winds, was copied into fv_io.F90 to prevent a circular dependency. It would be nice if it only needed to be defined once in the dynamical core. A logical place for it to live that would not result in a circular dependency would be fv_grid_utils.F90. This is where the cubed_to_latlon subroutine already lives, which does the reverse operation, converting cubed sphere D-grid horizontal winds to lat/lon A-grid horizontal winds.