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. Improve accuracy by using `float`, instead of `uint32_t` for `dutycycle`. Check [Change Duty Cycle #1](khoih-prog/ESP8266_PWM#1 (comment))
3. DutyCycle to be optionally updated at the end current PWM period instead of immediately. Check [DutyCycle to be updated at the end current PWM period #2](khoih-prog/ESP8266_PWM#2)
4. Optimize library code by using `reference-passing` instead of `value-passing`
5. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project
6. Update examples accordingly

Author: khoih-prog

README.md

@@ -272,6 +272,7 @@ void setup()
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #include <SimpleTimer.h>              // https://github.com/jfturcot/SimpleTimer

examples/ISR_16_PWMs_Array/ISR_16_PWMs_Array.ino

@@ -24,6 +24,7 @@
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #define LED_OFF             HIGH

examples/ISR_16_PWMs_Array_Complex/ISR_16_PWMs_Array_Complex.ino

@@ -24,6 +24,7 @@
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #include <SimpleTimer.h>              // https://github.com/jfturcot/SimpleTimer

examples/ISR_16_PWMs_Array_Simple/ISR_16_PWMs_Array_Simple.ino

@@ -24,6 +24,7 @@
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #define LED_OFF             HIGH

examples/ISR_Changing_PWM/ISR_Changing_PWM.ino

@@ -24,6 +24,7 @@
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #define LED_OFF             HIGH

examples/ISR_Modify_PWM/ISR_Modify_PWM.ino

@@ -24,6 +24,7 @@
 
 #define USING_MICROS_RESOLUTION       true    //false 
 
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
 #include "nRF52_MBED_Slow_PWM.h"
 
 #define LED_OFF             HIGH