Added DEPRECATED versions of statistic and variance in streaming MMD

DEPRECATED versions are available with

 - statistic type S_UNBIASED_DEPRECATED
 - null variance estimation method NO_PERMUTATION_DEPRECATED
 - null approximation method MMD1_GAUSSIAN_DEPRECATED
@sejdino Well, using DEPRECATED blockwise statistic computation, the kernel is computed on different samples compared to present methods. So apart from the normalizing constant, the statistic changes too. Should I be using deprecated statistic for covariance computation? The results won't exactly be the same with earlier implementation based on the current implementation we have because when using `compute_blockwise_statistic_DEPRECATED`, we split the data in halves for each p and q for all the samples streamed in *current burst*. but for covariance, I am presently streaming twice as many blocks in each burst when in earlier implementation it just used to stream the same number of samples split in halves. with present implementation, this change didn't affect result because computation is done block-wise instead of burst-wise like before. I need to change it a bit then.