Merge branch 'fix_slope' into develop (PR #196)

Merge branch 'fix_slope' into develop (PR #196)

This merge corrects the calculation of df/dx and df/dy for subgrid fields
and for nested domains in geogrid.

TYPE: bug fix

KEYWORDS: subgrid, fire, terrain, slope, gradient

SOURCE: Timothy W. Juliano and Branko Kosovic (NCAR/RAL), Angel Farguell (San
Jose State University)

DESCRIPTION OF CHANGES:
Problem:
When processing terrain data for the WRF-Fire subgrid (variable ZSF) in geogrid,
the terrain gradients in the x- and y-directions (variables dzdxf and dzdyf) are
computed from the ZSF field. At present, the geogrid/src code
(process_tile_module.F) uses the meteorological grid cell spacing from the
outermost domain, rather than the subgrid (fire mesh) cell spacing from the
present domain, to compute the gradients. The resultant dzdxf and dzdyf fields
are computed correctly if subgrid_ratio_x=subgrid_ratio_y=1 and if max_dom=1;
however, if subgrid_ratio_x>1 on the fire mesh and/or max_dom>1 and the fire
mesh is in a child domain, then the resultant dzdxf field is computed
incorrectly and is an underestimation of the actual terrain gradient. The issue
is similar for the dzdyf field when subgrid_ratio_y>1. As the subgrid_ratio
increases, the underestimation becomes more pronounced.

Solution:
Use the present domain meteorological grid cell spacing and account for the
subgrid_ratio when computing the terrain gradients. This simply requires calling
the get_domain_resolution subroutine to pull the present grid cell spacing of
the meteorological domain, passing in the respective subgrid_ratio to the
calc_dfdx and calc_dfdy subroutines, and dividing the correct meteorological
grid spacing by subgrid_ratio.

LIST OF MODIFIED FILES:
M geogrid/src/process_tile_module.F

TESTS CONDUCTED:
Run geogrid before and after the code modifications and inspect variables dzdxf
and dzdyf to ensure magnitudes are correct compared to the source SRTM data.

RELEASE NOTE:
When processing terrain data for the WRF-Fire subgrid, terrain gradient
variables dzdxf and dzdyf are computed using the meteorological grid cell
spacing from the outermost domain, thus resulting in an underestimation in
gradient magnitude when max_dom>1 or the subgrid_ratio for the fire mesh is
greater than 1. Therefore, we correctly account for the current domain
resolution and the subgrid_ratio when computing the terrain gradient variables.

* fix_slope:
  Fixing domain dx and dy in calc_dfdx and calc_dfdy
  Fixing bug in calc_dfdx and calc_dfdy