Renamed enable/disable_output so that they can, if really necessary, be
used without going through channels
loopback test uses fewer pins so that they can be used to test interrupts and manual testing with button and LED
first stab at loopback test for differential channels
mark timerfd tests that were missing decorators
Loopback tests for PCF8591 differential inputs
PCF8591 can only measure up to 50% voltage differential, so
differential input channels return -0.5 to +0.5.