User EEPROM Compatibility

[bwjohns4]

This changes the previous version to instead of setting a persistent flag, users can just add their EEPROM_SIZE to the constructor which is 0 by default. Then if user EEPROM is 0, then call .begin(_offset + _size) but only if the EEPROM is not already begun. This is checked by EEPROM.length(). If the value is not zero then that means the user sketch has already called .begin() and we should call it within this library or it could reallocate the buffer and result in loss of information.

[krzychb]

@bwjohns4 I am sorry for the slow reply! Could you please check if the drawing below properly documents the changes?

eeprom-layout.drawio.zip