Control surface data input

[daviderovell0]

Further develop the simple potentiometer sample program (#8) to have an array of at least 4 potentiometers with data being read simultaneously (real time) from all 4.

Since the pots ADC uses SPI, the main idea is to write a loop that keeps switching SPI slave and reading each input in turn, fast enough to appear simultaneous. We might need to use /dev direct access programming instead of /sys library such as WiringPI to achieve higher speed.

[daviderovell0]

Implemented in #29