FIX missing class attribute in ICA. Common test was failing
DOC: some documentation for the Tree Cython structure
DOC copyedit RBM docstrings

Removed the fake LaTeX; it wasn't showing up on the website
and it looks bad in IPython.
Merge pull request #2216 from glouppe/tree-doc

[MRG] Additional documentation for the Tree cython structure.
MAINT moved the pls file based module to a folder
MAINT created a temporary class _CCA to avoid circular imports
FIX using the deprecating decorators made some tests fail.

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 removed the pls module
MAINT added the cca module
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
FIX one last _pls missing in the documentation...
FIX added the new module to the setup.py installation
FIX docstrings imported the CCA and PLS methods from unexisting modules
DOC improved the docs on cross_decomposition
MAINT deprecated the pls module, moved CCA to cca_