Integrates new knn-engine in knndm and nndm

[JanLinnenbrink]

This PR integrates the new distance engine in knndm and nndm and thereby removes the dependency of FNN. Also, it aligns the parameters of nndm with those in knndm, introduces Mahalanobis distances to nndm and updates the documentation of these functions. Lastly, it adds tests that were missing in nndm (e.g., Mahalanobis distance).

[goergen95]

Great, thanks! Good to see that the distance engine also works in that part of the package. Zooming into knndm_geo() and knndm_feature() it is still hard to make out what is happening in the central places and there is still some code duplication. Would you be down to working on a further refactor of those two functions in the next days? Otherwise I will take it on once I am back in the office.