Rename variable `extr_date` to ifield

Because we are going to use it for another field.

Inst_eddies/Tests/examine_eddy.f90

@@ -28,7 +28,7 @@ program examine_eddy
   TYPE(shpfileobject) hshp
   character(len = :), allocatable:: slice_dir, slice_o_dir
   type(eddy) e
-  integer extr_date ! field index in DBF file
+  integer ifield ! field index in DBF file
 
   integer, allocatable:: ishape_last(:) ! (d0:)
   ! shape index (0-based) in the collection of shapefiles of the last
@@ -88,8 +88,8 @@ program examine_eddy
 
   call shp_open_03(hshp, pszshapefile = slice_o_dir // "/extremum", &
        pszaccess = "rb")
-  call dbf_get_field_index_03(hshp, "date", extr_date)
-  call dbf_read_attribute_03(d0, hshp, extr_date, ishape = 0)
+  call dbf_get_field_index_03(hshp, "date", ifield)
+  call dbf_read_attribute_03(d0, hshp, ifield, ishape = 0)
   call read_column(ishape_last, file = slice_o_dir // "/ishape_last.txt", &
        my_lbound = d0)