[MICRO] INTERNAL_REFERENCE on negative input not implemented.

[dsankouski]

analogComp.cpp file, line 35 states: "//for Atmega32U4, only ADMUX is allowed as input for AIN-"

But that seems not true, as ATmega32U4 datasheet, section 23.2 "Analog Comparator Multiplexed Input"(p.295) states: "If ACME is cleared or ADEN is set, the Bandgap reference is applied
to the negative input to the Analog Comparator."