port 07081a7590bc80a65d0a3f9a6fd6fa3f3b16e777 from Pd-l2ork: introduced potential fix for spi issues on RPi3

*added new externals for raspberry pi (disis_gpio for digital I/O and disis_spi for analog I/O using mcp3008 A/D converter). Provided preliminary help files. PWM on gpio needs to be tested further. So does the build script. TODO: add this to the installer, including 4755 chown_gpio script.

initial commit of the custom raspberry_pi externals supporting read/write from digital pins, analog pins (using shield with a mcp3008 A/D converter), and netsend~/netreceive~ optimized for rpi (net objects being copies of work done in the core pd)