Multithreading and performance

This release mainly includes performance improvements as well as small bug fixes. The tabulation of the correlation functions now supports using multiple threads. Additionally, TabCorr now only saves those correlation functions for which halo pairs can in principle be found. This saves a few percent of disk and memory space and improves performance. Additionally, predicting the galaxy correlation functions is now significantly faster by a factor of around 2. Finally, a bug was fixed where the contributions of a small number of halos to the galaxy correlation function was missed. In the all the test performed, this lead to an insignificant change in the predicted correlation function.