Updated info on Turntables pages

Added some information regarding modeling railroad turntables.

First, I added some back-of-the-envelope calculations to the Railroad Turntable page to verify that the maximum speed of my planned stepper motor will not be exceeded. Also, on the Turntable Calculations page I edited a note regarding Adafruit replacing their 16:1 geared stepper motor with a 64:1 model. As of November 2021 they have reversed that change, so the 16:1 motors are again being sold. Based on the maximum-speed calculations I mentioned above, this is a very good thing. 😉