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/multiFileProject/multiFileProject.h

@@ -0,0 +1,24 @@
+/****************************************************************************************************************************
+  multiFileProject.h
+  For Teensy boards
+  Written by Khoi Hoang
+
+  Built by Khoi Hoang https://github.com/khoih-prog/Teensy_TimerInterrupt
+  Licensed under MIT license
+  
+  Now even you use all these new 16 ISR-based timers,with their maximum interval practically unlimited (limited only by
+  unsigned long miliseconds), you just consume only one SAMD timer and avoid conflicting with other cores' tasks.
+  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.
+*****************************************************************************************************************************/
+
+// To demo how to include files in multi-file Projects
+
+#pragma once
+
+// Can be included as many times as necessary, without `Multiple Definitions` Linker Error
+#include "RPi_Pico_TimerInterrupt.h"
+
+// Can be included as many times as necessary, without `Multiple Definitions` Linker Error
+#include "RPi_Pico_ISR_Timer.hpp"