Do not test whether `e%max_speed` is NaN

It can never happen.

Inst_eddies/write_eddy.f90

@@ -6,15 +6,13 @@ contains
 
   subroutine write_eddy(e, hshpc, date, i)
 
-    use, intrinsic:: ieee_arithmetic, only: ieee_is_nan
-
     ! Libraries:
     use jumble, only: pi, rad_to_deg, assert
     use shapelib, only: shpt_polygon, shpfileisnull
     use shapelib_03, only: shp_append_point_03, dbf_write_attribute_03, &
          shp_append_object_03, shp_append_null_03
 
-    use derived_types, only: eddy, missing_speed, shpc_slice_handler
+    use derived_types, only: eddy, shpc_slice_handler
 
     type(eddy), intent(in):: e
     TYPE(shpc_slice_handler), intent(in):: hshpc
@@ -36,15 +34,8 @@ contains
     call dbf_write_attribute_03(hshpc%extremum, ishape, hshpc%extr_date, date)
     call dbf_write_attribute_03(hshpc%extremum, ishape, hshpc%extr_eddy_index, &
          i)
-
-    if (ieee_is_nan(e%max_speed)) then
-       call dbf_write_attribute_03(hshpc%extremum, ishape, hshpc%extr_speed, &
-            missing_speed)
-       ! (Cannot write NaN to dbf file.)
-    else
-       call dbf_write_attribute_03(hshpc%extremum, ishape, hshpc%extr_speed, &
+    call dbf_write_attribute_03(hshpc%extremum, ishape, hshpc%extr_speed, &
             e%max_speed)
-    end if
 
     if (.not. shpfileisnull(hshpc%extr_proj)) then
        call shp_append_point_03(ishape, hshpc%extr_proj, &