Fixed math

This is why I wanted to sleep on the code I wrote while falling asleep
rather than immediately submitting a pull request.

Marlin/Marlin.h

 extern bool axis_relative_modes[];
 extern int feedmultiply;
 extern int extrudemultiply; // Sets extrude multiply factor (in percent)
-extern float filament_area[EXTRUDERS]; // cross-sectional area of filament (in cubic millimeters)
+extern float volumetric_multiplier[EXTRUDERS]; // reciprocal of cross-sectional area of filament (in square millimeters), stored this way to reduce computational burden in planner
 extern float current_position[NUM_AXIS] ;
 extern float add_homeing[3];
 #ifdef DELTA

Marlin/Marlin_main.cpp

 int feedmultiply=100; //100->1 200->2
 int saved_feedmultiply;
 int extrudemultiply=100; //100->1 200->2
-float filament_area[EXTRUDERS] = {1.0
+float volumetric_multiplier[EXTRUDERS] = {1.0
   #if EXTRUDERS > 1
     , 1.0
     #if EXTRUDERS > 2
           SERIAL_ECHOLN(tmp_extruder);
           break;
         }
-        filament_area[tmp_extruder] = area;
+        volumetric_multiplier[tmp_extruder] = 1 / area;
       }
       break;
     case 201: // M201

Marlin/planner.cpp

 #endif
   block->steps_z = labs(target[Z_AXIS]-position[Z_AXIS]);
   block->steps_e = labs(target[E_AXIS]-position[E_AXIS]);
-  block->steps_e *= filament_area[active_extruder];
+  block->steps_e *= volumetric_multiplier[active_extruder];
   block->steps_e *= extrudemultiply;
   block->steps_e /= 100;
   block->step_event_count = max(block->steps_x, max(block->steps_y, max(block->steps_z, block->steps_e)));
     delta_mm[Y_AXIS] = ((target[X_AXIS]-position[X_AXIS]) - (target[Y_AXIS]-position[Y_AXIS]))/axis_steps_per_unit[Y_AXIS];
   #endif
   delta_mm[Z_AXIS] = (target[Z_AXIS]-position[Z_AXIS])/axis_steps_per_unit[Z_AXIS];
-  delta_mm[E_AXIS] = ((target[E_AXIS]-position[E_AXIS])/axis_steps_per_unit[E_AXIS])*filament_area[active_extruder]*extrudemultiply/100.0;
+  delta_mm[E_AXIS] = ((target[E_AXIS]-position[E_AXIS])/axis_steps_per_unit[E_AXIS])*volumetric_multiplier[active_extruder]*extrudemultiply/100.0;
   if ( block->steps_x <=dropsegments && block->steps_y <=dropsegments && block->steps_z <=dropsegments )
   {
     block->millimeters = fabs(delta_mm[E_AXIS]);