revise MRAA driver's GPIO implementation

[2bndy5]

resolves #965

[TMRh20]

Tested on RPI Model B

[2bndy5]

It also still works on my RPi3. My RPi4 is still exiting with:

terminate called after throwing an instance of 'std::invalid_argument'
  what():  Invalid GPIO pin specified
Aborted

[2bndy5]

using sudo journalctl, I see the logs:

Mar 23 10:34:15 rpi4-64bit libmraa[823]: libmraa version v2.2.0-49-g258bfcf initialised by user 'brendan' with EUID 1000
Mar 23 10:34:15 rpi4-64bit libmraa[823]: gpio: platform doesn't support chardev, falling back to sysfs
Mar 23 10:34:15 rpi4-64bit libmraa[823]: libmraa initialised for platform 'Raspberry Pi 4 Model B' of type 5
Mar 23 10:34:15 rpi4-64bit libmraa[823]: gpio22: init: Failed to write to 'export': Invalid argument
Mar 23 10:34:15 rpi4-64bit kernel: export_store: invalid GPIO 22

gpio: platform doesn't support chardev, falling back to sysfs

This seems like a problem mraa needs to fix. I'm confident now that these changes are not breaking it even more on RPi4+.