Raise exception on non-unique column index in to_hdf for fixed format.

Fixes #7761.
API: Add PeriodIndex.resolution
BUG: is_superperiod and is_subperiod cannot handle higher freq than S
Docs: Panel.dropna now exists, update docs accordingly.
Merge pull request #7803 from sinhrks/superperiod

BUG: is_superperiod and is_subperiod cannot handle higher freq than S
CLN: Simplify LinePlot flow
TST: add mixed-type panels & frames to generate_legacy_pickles

Don't generate them for pre-0.14.1 as they cannot be unpickled.
BUG: fix unpickling of some pre-0.14.1 pickles with non-unique items

Series, frames and panels that contain only one block can be unpickled
under the assumption that block items correspond to manager items 1-to-1.
TST: add tests for GH 6572
BUG: Fix for multiple ints as levels in DataFrame.stack()

Fix for multiple level numbers

If passed levels match level names, use them directly

Add test cases for multiple stacking

Add check for out of range negative level num in _get_level_number()

Use _get_level_number() to do validation and conversion

Explain why level list isn't iterated over directly

Raise exception on fall-through case

Add test cases for raising exceptions

Use _get_level_number() to convert and validate negative levels

Add changes to release notes

Fix Python 2.6 build issue: use assertRaisesRegexp from testing module

Add tests for out of bounds level numbers

Move API change note to correct section

Add blank line after if/elifs

Add blank line between sections

Too many blank lines between functions

Add multiple stacking examples

Reference to new examples in What's new docs

More blank lines between sections

Remove unused itertools import
BUG: timeseries subplots may display unnecessary minor ticklabels
Merge pull request #7788 from filmor/patch-2

Raise exception on non-unique column index in to_hdf for fixed format.
Merge pull request #7794 from immerrr/fix-unpickling-series-with-non-unique-index

BUG: fix reading pre-0.14.1 pickles of containers with one block and dup items
Merge pull request #7770 from onesandzeroes/int-stacking

BUG: Fix for passing multiple ints as levels in DataFrame.stack() (#7660)
Merge pull request #7810 from sinhrks/test_gh6572

TST: add tests for GH 6572
Merge pull request #7708 from sinhrks/dtmixin_reso

API: Add PeriodIndex.resolution
DOC: edits in v0.15.0.txt
DOC: add sub-levels to reshaping.rst
Merge pull request #7717 from sinhrks/lineplot_cln

CLN: Simplify LinePlot flow
Merge pull request #7801 from sinhrks/axlayout_bug

BUG: timeseries subplots may display unnecessary minor ticklabels