Fix compiler errors when compiling for Arduino Nano 33 BLE platform

[NeilBaner]

My fix for #20.

I renamed a couple of variables D1 and D2 as they were causing a conflict with certain Arduino headers for the Nano 33 BLE. I also added a cast-to-integer in some of the calls to Wire::write, as the new mbed-based implementation of Wire.h provides two candidates, write(uint8_t) and write(char*).

I tried my best to make sure this code doesn't break anything else. I tried compiling for a few different Arduino platforms, including: Arduino Uno, Arduino Nano, Arduino Nano 33 BLE, Arduino Nano Every, Arduino Portenta H7, and Arduino Nano 33 IOT, and the library compiled without any issues. I apologise for not knowing the proper procedure to test my code thoroughly, I am a student and very new to this.

[NeilBaner]

I apologise for not knowing the proper procedure to test my code thoroughly, I am a student and very new to this.

I notice there's a CI check which does pass, so I guess that's good?