(wip) [cluster] Complete variational Bayes and VBmerge likelihood bound

o bound computation stable but doesn't always increase => test case fails
o doc includes default values, dimensions of arrays
o split into E- and M-step, perform E step first to allow consistent bound computation
o regularize responsibility to avoid overflows in log(0)
o simplify prune with getattr
o _expect_log_p_Z simplified
o E step and M step identical except for overloading
o return mode of variational posterior as result
o remove clumsy (and false) caching of intermediate constants

Bug fixes:
o m0[k] -> m0
o second part of covariance added to _update_W
o fix signs in bound
o user right Dirichlet B and C
o alpha0 vector and scalar disentangled