[ COMMAND ] new commands SBASE, SREG, SCHAN

SBASExx changes I2C slave address for current channel to be xx
SREGxx=yy writes yy to register xx
SCHANx changes I2C bus channel(x = 0, 1, 2 or 3)
[ CLASS ] serial_i2c class update

confirms isUse flag on all member functions; passes if not in use
can set I2C slave address for each channel
can write data on single register
test

test for new commands