Marble Clock RTC

This is a redesign of the electronics for the Marble Clock "Time Rolls On" by HFoG on Thingiverse: https://www.thingiverse.com/thing:4891112

Follow the instructions there to print and assemble the printed parts of this clock.

I did add an "Open Shell 2.stl" which provides an alternate for the electronics enclosure used in the original. Use this or shell.stl as suits your taste. I like to see the electronics.

This version adds a DS3231 real-time clock (RTC) module which uses a crystal oscillator to keep time instead of using the Arduino Nano built-in oscillator for timing. I used a DS3231SN which has a temperature-stabilized crystal oscillator but any DS3231 should work.

Adding the RTC requires three connections to the Nano, plus +5V power and ground. The SDA and SCL pins on the SD3231 connect to the Arduino I2C pins (pins A4 and A5 on the Nano) for control, and the SQW (square wave output) pin connects to the D3 pin on the Nano to provide a once-per-second interrupt for