added doc remark for returning pixel_coordinates

[theo258]

Added a remark that the first entry in the returning pixel_coordinates is always zero to match the index of the pixels (c.f. #93)
(My first pull request so correct me if something went wrong.)

[coveralls]

Coverage remained the same at 99.453% when pulling 96dd851 on theo258:master into 81fff8b on jni:master.

[theo258]

Is there a reason why the pixel_coordinates array is returned as float? As these values can only be integers one could save them as int by changing pixel_indices = np.concatenate(([[0.] * ndim],... to pixel_indices = np.concatenate(([[0] * ndim],... in csr.py. I'll create another pull request if that sounds useful.

[jni]

Gah, I'm sorry I missed this for so long, @theo258! Thank you very much for this tweak!

Is there a reason why the pixel_coordinates array is returned as float?

Yes, unique_junctions= replaces junctions containing more than one pixel with their centroid, which can be float. This is described in detail in the paper supplementary info, but should probably go in the docs. I'm still not sure about best approaches here, and suggestions are very welcome!