BUG/CLN: MI now checks level & label compatibility

Via verify_integrity kwarg. Internals all pass False.

CLN: Verify integrity by default on label/level setting
Merge pull request #5214 from jtratner/mi-validate-level-label-compat

BUG/CLN: MI now checks level & label compatibility
CLN: Add _reset_cache method to PandasObject
CLN/ENH: Make names getattr'able in core/ops +PEP8.

Now the name passed to a method can be used to get that method from the
NDFrame (previously you had things like `or_ [|]`. better to just have
them all be named exactly what they are s.t. subclasses can do
`getattr(super(self, MyClass), name)(*args, **kwargs)`.
Fixed assertion error message
DOC: fix building of gbq docs
Merge pull request #5242 from ojh/master

Fixed assertion error message
Merge pull request #5241 from jorisvandenbossche/docs-gbq-fixes2

DOC: fix building of gbq docs
API: default export for to_clipboard is now csv (GH3368)
Merge pull request #5070 from jreback/clip

API: default export for to_clipboard is now csv (GH3368)
TST: Added test for Excel writers with duplicated column names.

Test for issue #5235
Merge branch 'tst_excel_duplicated_cols' of https://github.com/jmcnamara/pandas into jmcnamara-tst_excel_duplicated_cols
API: add excel kw to to_clipboard to preserve compat behavior (related GH5070)
Merge pull request #5243 from jreback/clip2

API: add excel kw to to_clipboard to preserve compat behavior (related GH5070)
Merge pull request #5240 from jtratner/cleanup-core_ops

CLN/ENH: Make names getattr'able in core/ops +PEP8.
Merge pull request #5238 from jtratner/add-_reset_cache-to-PandasObject

CLN: Add _reset_cache method to PandasObject
Make doc compiling under windows workable

The doc generation failed under windows due to problems with sphinx
and encoded umlauts in code (see links in
https://github.com/pydata/pandas/issues/5142).

The workaround is to replace the offending text with one which does not fail
(but which makes the example a bit pointless), build the docs and restore the
old text.
Remove leftover files from doc builds

On windows, these generated files are often left over due to
"file in use" errors. Remove them after the doc build has finished.