let user retrieve the location of the dip

[JohannesBuchner]

A shot at #76

There are a few issues with the PR:

• no tests
• only very brief doc in the python and C++ function for the new parameter
• data type is weird, it should be a single number -- it's been a while since I used C++; I copied the code from other objects, which use arrays, so this is a 1-element array.
• potentially off by one bug, one in the --dipidx and one in the subtracting one at the end.

Would be cool to have this feature supported in diptest though!

[RUrlus]

Ha @JohannesBuchner, thanks for taking a stab at it. I ran into some other things I wanted to fix at the same. I'm took your commit and went from there.
I don't have permission to push to your branch so I'll close this one and continue in #78

[JohannesBuchner]

Great, thanks for picking this up