This PR completes all docstrings for the imio module.

All the docstrings in the module now adhere to the [NumPy docstring guidelines](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt).

The function `raveler_to_labeled_volume` now takes named arguments instead of the generic `**kwargs`. I have checked all other files to ensure no calls to this function are affected.