add streaming mmd, refactor linear time mmd

@karlnapf Please have a look at this class structure. Added a base class CStreamingMMD to make things easier for CLinearTimeMMD and CBTestMMD and refactored CLinearTimeMMD. Gaussian approximation and multiple kernel stuff readily works for both, and moved the parts common to both inside the base. It provides a method for stream data blocks and an interface for computing squared MMD which is to be defined by the subclasses.

Tested all existing unit-tests and integration tests and locally everything seems to work as before.