Move mean/linspace from SGVector to CStatistics/CMath respectively

@iglesias Turned out that `SGVector::linspace_vec` was being used at one place, so I didn't remove it.
you want a seperate test case with a name that roughly describes it instead. Keep unit tests as atomic as possible.
Also, I suggest you allocate a vector of float64 numbers, which are all set to the maximum that float64_t can take. And then compare whether their average is also that maximum 64bit float number