Add more statistics attributes to ListTag

[MusicScore]

I'm thinking we should add support for finding the median, mode, and standard deviation of a list of numbers.

Optionally, we can also add support for significant figures? For example, <element[15532].significant_figures[4]> would result in 15530. I figure it would make life a little easier for people who might want to use significant figures purely for aesthetic purposes (for example, when a decimal results in 1.3 but the user wants the output to have a few extra zeroes for some particular reason). Of course, this particular use case can be remedied with ElementTag.pad_right, but it would be less of a pain if users could just use a built-in tag to handle the digit-counting logic instead.