Add functions to get working and user directory.

[frankenjoe]

In #155 we implement a function that returns the directory of the current script. It might be a good fit to also add a function that returns the current working directory (e.g. working_dir()) and user directory (e.g. user_dir()).

[hagenw]

The current working dir is returned by os.getcwd(), or are you referring to a different directory?

[frankenjoe]

No, audeer.working_dir() would return os.getcwd() and audeer.user_dir() return audeer.path("~"). Then we provide functions for the three special directories. But maybe that is not really needed.

[hagenw]

It is not really needed as there exist alternatives that involve only a single function call (as was not the case with audeer.script_dir()).
But I see the point for providing these functions for symmetric reasons ;)

audeer.user_dir() has the problem, that it is not completely obvious what is meant. As I first thought you are referring to the directory in which the user was when starting a script. So maybe audeer.home_dir() would be a better choice?

[frankenjoe]

Right, if we decided to add these functions then audeer.home_dir() would be a better fit.

[frankenjoe]

which the user was when starting a script.

Mhh, if it's possible to get this location, this would also be a nice function to have :)