Support up to 4 mod-IOs

See merge request !43

coupler/mod_io_i2c.h

 // global relay state - XXX: use a list rather than individual variables
 uint8_t I2C_0_RELAYS_STATE = 0; // state of 4 relays at I2C slave 0
 uint8_t I2C_1_RELAYS_STATE = 0; // state of 4 relays at I2C slave 1
+uint8_t I2C_2_RELAYS_STATE = 0; // state of 4 relays at I2C slave 4
+uint8_t I2C_3_RELAYS_STATE = 0; // state of 4 relays at I2C slave 3
 
 // the default addresses of MOD-IOs
 static char *DEFAULT_I2C_0_ADDR = "0x58";