DCC_DEBUG: new function getNestedIrqCount()

getNestedIrqCount() returns how often the DCC-Irq interrupted itself.
This is a hint that the DCC Signal is not clean and there are glitches.

NmraDcc.h

@@ -203,6 +203,7 @@ class NmraDcc
 	uint8_t getTickCount(void);
 	uint8_t getBitCount(void);
 	uint8_t getState(void);
+  uint8_t getNestedIrqCount(void);
 #endif
 
 };