TST add tests for nd grid-search can train_test_split
MAINT dev version to follow PEP440
FIX correct name due to rebase error
Spelling in heading newgroups -> newsgroups

The section title is "newgroups", this fixes it so that it matches the rest of the document.
Merge pull request #4003 from ClashTheBunny/patch-1

DOC Spelling in heading newgroups -> newsgroups
ENH Added method_filter to all_estimators function.

method_filter can be used to filter the estimators based on the existence of one or more methods passed as a list of method names.

The default method_filter value is None.

It can also be an empty list.
FIX fix the removal of set while resolving merge conflicts

COSMIT minor pep8 fixes in sklearn.utils.testing
TST Added partial_fit estimator checks

* Test for estimator restart upon fit
* Validation of classes argument
* Test to check that partial_fit returns reference to self
* Refactor multiple imports from utils.testing into single import
TST Added tests for estimators implementing partial_fit
ENH method_filter will now accept string or list of strings

ENH method_filter/type_filter allows prefixing filter names with !
    to skip those particular methods/types.

ENH type_filter will skip if the list is empty

COSMIT minor cosmetic and pep8 fixes
TST Added tests for all_estimators function
FIX multiple issues in the newly added partial_fit tests.