MAINT moved the pls file based module to a folder
TEST fixing pls tests failing:

Because sklearn classifier need not to use *args and **wargs, we cannot use decorators on init class
FIX CCA and _CCA need to be skipped on a bunch of tests because the code is very unreliable
MAINT Move the pls to the cca to a cross_decomposition module
MAINT renamed pls to cross_decomposition in the documentation
FIX the example plots of the pls module did not import pls methods from the correct module
FIX removed the cca and pls modules

one last _pls missing in the documentation...
FIX added the new module to the setup.py installation
DOC improved docs/docstrings on cross_decomposition
MAINT deprecated the pls module, moved CCA to cca_

TEST don't need to excluse _PLS and _CCA
FIX init methods of ABCMeta class also need to be abstract
Merge pull request #2182 from NelleV/pls_refactor_2

[MRG] Small refactoring of the PLS module.