[cluster] Improve tests and bugfix variational

o fix init of nu
o regularize N_comp to avoid inf
o replace some loops with einsum
o rigorous test of variational bayes (not VB merge)
[cluster]  - removed __iter because python automatically does what
             is written there when __getitem__ is defined
[cluster] Fix two bugs in variational

o independent copy of inv_W0
o check for improper precision matrices
o ensure symmetric positive definite sample covariance
o add 'slow' attribute to a unit test
(wip)[variational] Improve stability and usability of variational

o PEP8, default values in doc
o equation numbers to implementations
o add E step at the end of prune()
o add run() method that does convergence checking etc.
o rewrite _update_log_rho() stepwise for better stability and in-place modification
o reset prune test case to old init of nu, fails. Have to understand why.