partial_fit for scalers

doc/modules/preprocessing.rst

doc/whats_new.rst

sklearn/decomposition/incremental_pca.py

sklearn/preprocessing/data.py

sklearn/preprocessing/tests/test_data.py

sklearn/utils/extmath.py

sklearn/utils/sparsefuncs.c

sklearn/utils/sparsefuncs.py

sklearn/utils/sparsefuncs_fast.c

sklearn/utils/sparsefuncs_fast.pyx

sklearn/utils/testing.py

sklearn/utils/tests/test_extmath.py

sklearn/utils/tests/test_sparsefuncs.py