Deinitialize the pin definition

Without freeing the DAC, the pin will continue to be configured as DAC. Which make it impossible to change it from Analogout to anything else. 
Check this code that allow you to change the AnalogOut to DigitalOut

<<code>>
#include "mbed.h"

class myAnalog : public AnalogOut{
    public:

    myAnalog(PinName myname);
    ~myAnalog();
    
 PinName _myname;        
};
    
myAnalog::myAnalog(PinName myname):AnalogOut(myname),
                                   _myname(myname){
                                       ;
                                   
}
 myAnalog::~myAnalog(){
   analogout_free(&this->_dac);
 }

myAnalog *x=0;
DigitalOut *xx=0;
void do_something_analog() {
    x=new myAnalog(PA_4);
    double g=0.0;
    while(g<0.5){
    x->write(g);
    g=g+0.1;
    wait_ms(50);
    }
     if (x!=0){
      delete x;
      x=0;
    }
}

void do_something_digital() {
    xx=new DigitalOut(PA_4);
    for(int i=0;i<10;i++){
        *xx = 1;
       wait_ms(50);
        *xx = 0;
        wait_ms(50);
    }
   if (xx!=0){
      delete xx;
      xx=0;
    }
}

int main()
{
    printf("\nAnalog loop example\n");
    while(1) {
    do_something_digital();
    do_something_analog();    


 }
}

<</code>>

Author: mjm2017

drivers/AnalogOut.h

@@ -138,7 +138,8 @@ class AnalogOut {
 
     virtual ~AnalogOut()
     {
-        // Do nothing
+        // deinitialize the pin configuration totally. This should allow chaining the definition of the pin
+          analogout_free(&this->_dac);
     }
 
 protected: