Improved performance & better whitening

* Bugfix: there was a math error in the whiten function. The result is small for the tests I've computed, but this version is now exact. (Thanks to Adrian Shelley for this)
* Performance: as a result, the performance of whiten is much higher
* Performance: if only ZZ is to be computed, the traces are sliced and pre-whitened, which makes a huge difference for large number of pairs (but requires much more RAM !)

TODO:

* Allow to manually disable the "fast mode" (= pre-whitening) if the RAM is an issue
* Allow to define the cosTaper lengths in the general configuration (currently hard-coded), for compute_cc and whiten