Cleanup of temperature code

- Reduce calls to millis()
- General cleanup of manage_heaters
- General cleanup of pid autotune
- Formatting here & there
- Macros to clean up and shrink ISR code (reduced by ~364 lines)

Marlin/pins_RUMBA.h

@@ -6,6 +6,10 @@
   #error Oops!  Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu.
 #endif
 
+#if EXTRUDERS > 3
+  #error RUMBA supports up to 3 extruders. Comment this line to keep going.
+#endif
+
 #define X_STEP_PIN         17
 #define X_DIR_PIN          16
 #define X_ENABLE_PIN       48