Make getters const (#56)

* add const for a getter
* make getAddress() const and inline
* move getAddress() to header file

Author: hasenradball

PCF8574.cpp

@@ -36,20 +36,12 @@ bool PCF8574::isConnected()
   return ( _wire->endTransmission() == 0);
 }
 
-
 bool PCF8574::setAddress(const uint8_t deviceAddress)
 {
   _address = deviceAddress;
   return isConnected();
 }
 
-
-uint8_t PCF8574::getAddress()
-{
-  return _address;
-}
-
-
 //  removed _wire->beginTransmission(_address);
 //  with    @100 KHz -> 265 micros()
 //  without @100 KHz -> 132 micros()

PCF8574.h

@@ -36,8 +36,7 @@ class PCF8574
   //  note: setting the address corrupt internal buffer values
   //  a read8() / write8() call updates them.
   bool    setAddress(const uint8_t deviceAddress);
-  uint8_t getAddress();
-
+  uint8_t getAddress() const { return _address; }
 
   uint8_t read8();
   uint8_t read(const uint8_t pin);
@@ -54,7 +53,7 @@ class PCF8574
   uint8_t readButton8(const uint8_t mask);
   uint8_t readButton(const uint8_t pin);
   void    setButtonMask(const uint8_t mask) { _buttonMask = mask; };
-  uint8_t getButtonMask() { return _buttonMask; };
+  uint8_t getButtonMask() const { return _buttonMask; };
 
 
   //  rotate, shift, toggle, reverse expect all lines are output