Change from float to double (grackle seems to be broken with float)

configure.ac

@@ -533,7 +533,7 @@ if test "x$with_grackle" != "xno"; then
       [grackle],
       [initialize_chemistry_data],
       [AC_DEFINE([HAVE_GRACKLE],1,[The GRACKLE library appears to be present.])
-        AC_DEFINE([CONFIG_BFLOAT_4],1,[Use float in grackle])
+        AC_DEFINE([CONFIG_BFLOAT_8],1,[Use double in grackle])
       ],
       [AC_MSG_ERROR(Cannot find grackle library!)],
       [$GRACKLE_LIBS $GRACKLE_INCS $FCLIBS]