[MRG+1] FIX unstable cumsum (#7376)

* FIX unstable cumsum in utils.random

* equal_nan = true for isclose
since numpy < 1.9 sum is as unstable as cumsum, fallback to np.cumsum

* added axis parameter to stable_cumsum

* FIX unstable sumsum in ensemble.weight_boosting and utils.stats

* FIX axis problem in stable_cumsum

* FIX unstable cumsum in mixture.gmm and mixture.dpgmm

* FIX unstable cumsum in cluster.k_means_, decomposition.pca, and manifold.locally_linear

* FIX unstable sumsum in dataset.samples_generator

* added docstring for parameter axis of stable_cumsum

* added comment for why fall back to np.cumsum when np version < 1.9

* remove unneeded stable_cumsum

* added stable_cumsum's axis testing

* FIX numpy docstring for make_sparse_spd_matrix

* change stable_cumsum from error to warning