Combine-pvalues-discrete

Several improvements to the documentation.

Several minor bugfixes and documentation improvements.

* Added `direction` service function to find out in which direction a two-sided combined *p* value leans.
* Fixed Wilcoxon’s signed-rank which broke due to a SciPy update.
* Some improvements to documentation.

* Implemented Wilcoxon’s signed-rank test.
* Implemented symmetrised variant of Edgington’s method.
* Various improvements to documentation.