Pass type to field.

pymagglobal/_commands.py

@@ -390,10 +390,7 @@ def maps(args):
     # BUGFIX: white edges in cartopy
     z_at[1] -= 180
     # use the core function to get the field
-    field = core.field(z_at, args.model)
-    # convert the field if necessary
-    if args.type == 'dif':
-        field = np.array(utils.nez2dif(*field))
+    field = core.field(z_at, args.model, field_type=args.type)
     # output formats for dif and nez components
     fmts = {'dif': ('%.1f', '%.1f', '%2.6f', '%2.6f', '%1.7e'),
             'nez': ('%.1f', '%.1f', '%1.7e', '%1.7e', '%1.7e')}