port a49a2bbc138b34080f36ad3d091e57db6bb0e7ba from Pd-l2ork: fixed segfault caused by the latest commit. Now everything should work including the RPi3.

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

port fa53423e426de1b0e54ba02f080b83b541d85b35 from Pd-l2ork: experimental update to disis_gpio and small bug fix to autotune

cleaned-up gpio external dependencies and supporting build script. made disis_gpio external into a comprehensive object for all things gpio, including pwm (both hardware and software) with sarcduino-like formatting and verbose output. The external requires sudo due to required access to /dev/mem and /sys/class/gpio

*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)