Documentation: AGVis Animations

[jinningwang]

@nparsly Nicholas, I've reorganized and improved the AGVis documentation, see #38. The online documentation is available at https://agvis.readthedocs.io/en/latest/. Given your and Nick's thesis, it might be helpful to give more detail in the Development section.

Animation calculation

In the Animations section, it might be helpful to explain more how the animation is made.

AGVis input data

In the Test Cases section, I made a clarification that the devices BusFreq are required in the test case to measure the Bus voltage, angle, and frequency.

AGVis logging info

It might also be helpful to have logging messages tell the variables' name, size, etc, that are used for visualization.