Merge pull request #889 from karlnapf/master

some first proper examples for kernel selection
re-added initialisation for current vector which prevents double free segfaults when streaming from fixed data in memory
moved SG_REF to a nicer place
removed prints
-removed KernelToCombinedKernel and replaced by a (proper) obtain_from_generic
-some better error messages
-added possibility to init with single features which are then applied to all subkernels (needed by MMD stuff)
some minor updates
mentioned Gaussian blobs generator
added combined kernel selection example