Merge pull request #847 from karlnapf/master

First working set of kernel selection methods
Merge pull request #848 from karlnapf/master

quickfix  for broken build
assed forgotten quare root, was wrong before
added single variance test
added maxmmd test for linear time MMD
removed a kernel init which is not necessary and causes problems with combined kernels
implemented compute_statistic(bool multiple_kernels)
added a maxmmd kernel selection test for the quadratic time mmd