Add code to measures module

[linuswagner]

measures needs some docstrings to ease the understanding of the methods for someone that works with the measures the first time.
Besides the basic argument explanation it would be helpful to find a link or a short explanation for each measure to get a feeling for what it does.
At the moment one has to go through the code in order to understand or guess the functionality, which makes the library harder to use and might lead to reimplementations in own projects of already existing code.

[linuswagner]

I'd assume, this is a task for @KonstantinWilleke or @sacadena (if others also see this as an issue. If not, feel free to close this issue)

[KonstantinWilleke]

Thanks Linus, you are quite right. All of these measures need docstrings, it's on my to-do list. I'll assign the issue to myself. In the long (hopefully not too long) term, these measures will go to mlutils, with docstrings and all.
In the meantime, feel free to ask me anything about the code there!