Add `step_deg` to `grid_nml`

We need this for the test `Extraction_eddies_periodic`, which serves
as input for the test `Successive_overlap_periodic`.

Inst_eddies/inst_eddies.f90

@@ -41,7 +41,7 @@ program inst_eddies
   ! the grid is global
 
   real lon_max, lat_max ! longitudes and latitudes, in degrees
-  real step(2) ! longitude and latitude steps, in rad
+  real step_deg(2), step(2) ! longitude and latitude steps, in degrees and rad
   logical periodic ! grid is periodic in longitude
   real time
 
@@ -77,7 +77,7 @@ program inst_eddies
   character(len = 200) iomsg
 
   namelist /main_nml/ min_amp, max_radius, min_radius
-  namelist /grid_nml/ corner_deg, nlon, nlat
+  namelist /grid_nml/ corner_deg, step_deg, nlon, nlat
 
   !--------------------------------------------------------------
 
@@ -127,8 +127,9 @@ program inst_eddies
 
   call assert(lon_max > corner_deg(1) .and. lat_max > corner_deg(2), &
        "inst_eddies coordinate order")
-  step = [(lon_max - corner_deg(1)) / (nlon - 1), &
-       (lat_max - corner_deg(2)) / (nlat - 1)] * deg_to_rad
+  step_deg = [(lon_max - corner_deg(1)) / (nlon - 1), &
+       (lat_max - corner_deg(2)) / (nlat - 1)]
+  step = step_deg * deg_to_rad
 
   ! As we are requiring the grid spacing to be uniform, the value of
   ! "periodic" may be deduced from the values of step(1) and nlon: