Changes

The variable gr now needs to be passed into subroutine pvertinterp
(which was formerly found in interpolation.F90) in order to generalize
the grid. This created a circular dependency, for grid_class references
interpolation, and now interpolation needed to reference grid_class
in order to use type grid. This road block prevented the model from
compiling. I moved subroutine pvertinterp to advance_clubb_core in
order to solve the problem.