revise MRAA driver's GPIO

[2bndy5]

The MRAA driver's GPIO implementation hasn't seen many updates for years. While it doesn't work well with newer RPi4 (and above), it could still get updated to conform with other drivers' GPIO implementations. The following will allow for a more robust and less-specific implementation of Arduino wrappers for the MRAA lib:

• cache the mraa::Gpio* instances
• make GPIO member functions static

Solution

I'll submit a PR as a proposal. Basically, just

• use a std::map<rf24_gpio_t, mraa::Gpio*> as a cache.
• convert class functions into static
• remove the non-static class (private) members:

  RF24/utility/MRAA/gpio.h

  Lines 29 to 33 in af7fbec

  	private:
  	int gpio_ce_pin; /** ce_pin value of the RF24 device **/
  	// int gpio_cs_pin; /** cs_pin value of the RF24 device **/
  	mraa::Gpio* gpio_0; /** gpio object for ce_pin **/
  	// mraa::Gpio* gpio_1; /** gpio object for cs_pin **/

• remove this:

  RF24/utility/MRAA/gpio.cpp

  Line 24 in af7fbec

  void GPIO::begin(uint8_t ce_pin, uint8_t cs_pin)

• cleanup this:

  RF24/RF24.cpp

  Lines 1033 to 1036 in af7fbec

  	#if defined(MRAA)
  	GPIO();
  	gpio.begin(ce_pin, csn_pin);
  	#endif

  and this:

  RF24/RF24.h

  Lines 130 to 132 in af7fbec

  	#if defined(MRAA)
  	GPIO gpio;
  	#endif

• address this:

  RF24/utility/MRAA/gpio.cpp

  Lines 39 to 40 in af7fbec

  	// WARNING: use of memory mapped file system is deprecated in MRAA lib
  	gpio_0->useMmap(true); // `false` (or just not calling `useMmap()`) uses default file system?

Additional context

I want this because I'm working to expose the GPIO and interrupt functions in the pyRF24 project, but the MRAA driver requires special context that seems unnecessary (in both RF24 lib and python bindings).

PS - I've already done much of this work in my CirquePinnacle lib, so it would be mostly copy-n-paste from there.