v1.2.0 to fix multiple-definitions linker error

### Releases v1.2.0

1. Fix `multiple-definitions` linker error. Drop `src_cpp` and `src_h` directories
2. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project.
3. Optimize library code by using `reference-passing` instead of `value-passing`
4. Update all examples

examples/TimerInterruptTest/TimerInterruptTest.ino

@@ -18,13 +18,6 @@
   The accuracy is nearly perfect compared to software timers. The most important feature is they're ISR-based timers
   Therefore, their executions are not blocked by bad-behaving functions / tasks.
   This important feature is absolutely necessary for mission-critical tasks.
-
-  Based on SimpleTimer - A timer library for Arduino.
-  Author: mromani@ottotecnica.com
-  Copyright (c) 2010 OTTOTECNICA Italy
-
-  Based on BlynkTimer.h
-  Author: Volodymyr Shymanskyy
 *****************************************************************************************************************************/
 /*
    Notes:
@@ -44,6 +37,7 @@
 #define TIMER_INTERRUPT_DEBUG         1
 #define _TIMERINTERRUPT_LOGLEVEL_     4
 
+// Can be included as many times as necessary, without `Multiple Definitions` Linker Error
 #include "RPi_Pico_TimerInterrupt.h"
 
 #ifndef LED_BUILTIN