add cython implementations of computationally expensive functions

[FlorianRingbeckIsea]

Dear Valdecy,

thanks a lot for providing this library. I noticed, that the import of large datasets can take some time. To improve the import performance, I implemented some critical functions in cython.

It would be great, if you could review, test and merge the code. Attached, you can also finde a small script to test the changes with a wos-dataset.

Best regards Florian test.zip