cosmetic: Changed all instances of nonlinear to non-linear
Merge pull request #2187 from FedericoV/non_negative_style

cosmetic: Changed all instances of nonlinear to non-linear
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