Add Planner::set_filament_size for 2.0.x parity

Marlin/Marlin_main.cpp

     // setting any extruder filament size disables volumetric on the assumption that
     // slicers either generate in extruder values as cubic mm or as as filament feeds
     // for all extruders
-    if ( (parser.volumetric_enabled = (parser.value_linear_units() != 0.0)) ) {
-      planner.filament_size[target_extruder] = parser.value_linear_units();
-      // make sure all extruders have some sane value for the filament size
-      for (uint8_t i = 0; i < COUNT(planner.filament_size); i++)
-        if (!planner.filament_size[i]) planner.filament_size[i] = DEFAULT_NOMINAL_FILAMENT_DIA;
-    }
+    if ( (parser.volumetric_enabled = (parser.value_linear_units() != 0.0)) )
+      planner.set_filament_size(target_extruder, parser.value_linear_units());
   }
   planner.calculate_volumetric_multipliers();
 }

Marlin/planner.h

     // Manage fans, paste pressure, etc.
     static void check_axes_activity();
 
-    static void calculate_volumetric_multipliers();
-
     /**
      * Number of moves currently in the planner
      */
 
     static bool is_full() { return (block_buffer_tail == BLOCK_MOD(block_buffer_head + 1)); }
 
+    // Update multipliers based on new diameter measurements
+    static void calculate_volumetric_multipliers();
+
+    FORCE_INLINE static void set_filament_size(const uint8_t e, const float &v) {
+      filament_size[e] = v;
+      // make sure all extruders have some sane value for the filament size
+      for (uint8_t i = 0; i < COUNT(filament_size); i++)
+        if (!filament_size[i]) filament_size[i] = DEFAULT_NOMINAL_FILAMENT_DIA;
+    }
+
     #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
 
       /**