error handling in the I2C communication

added a class I2CComm that handles the whole communication with the I2C
bus and sensors, but let the dev handle errors, error types, and do
something with it (more specifically, would be used within the SAT_*
sensor libs, coming in next commit).
corrected anomalies in the management of addresses

the address is read from EEPROM, and the assvAddress is set 0x04
(I2C_ADD_ASSV_1). in the previous version, the local address of the node
used for Wire.begin was 0x04.
refactoring to integrate I2CComm

refactored the SAT_Lum, SAT_Mag and SAT_Temp to use I2CComm class the
management of communications through I2C. This helps to make the multple
calls to Wire.begin() converge into a single place, and keep a
consistant local address of the node (that should be decided in
I2CComm).