ENH : add flag to box_plot and bxp to manage (or not) xticks

Closes #2921
 - added test for managa_xtick option to boxplot
 - added CHANGELOG and whats_new.rst entry for manage_xticks
TST : fixed duplicate test name
Avoid a null-pointer dereference in _tri.cpp

Found using the clang static analyzer:
if PyArray_ContiguousFromObject returns NULL,
PyArray_NDIM(x) causes a null-pointer dereference.
Initialize the states array with calloc in _macosx.m

To silence a warning about garbage data from the clang
static analyzer. Also check the calloc return value.
Release some unreleased objects in the macosx backend
Added 'interpolation_none_vs_nearest' example, without .DS_store files
Update interpolation_none_vs_nearest.py

Minor rewording to "interpolation_none_vs_nearest.py" description.
BUG/DOC : fix bad merge of INSTALL
Merge pull request #2994 from jkseppan/macosx-fixes

Minor fixes in _macosx.m
Merge pull request #2993 from jkseppan/tri-null-pointer

Avoid a null-pointer dereference in _tri.cpp
Update CHANGELOG
Update interpolation_none_vs_nearest.py

Changed the last few lines so that the auto-documentation build process correctly displays the results of the example.
Added a static version of 'None_vs_nearest-pdf.png'
Merge branch 'image-interpolation-example' of github.com:Stretch97/matplotlib into image-interpolation-example
Merge pull request #3002 from tacaswell/INSTALL_merge_fix

BUG/DOC : fix bad merge of INSTALL
Merge pull request #2995 from Stretch97/image-interpolation-example

Added 'interpolation_none_vs_nearest' example, without .DS_store files
DOC : edits to optional dependencies

 - cleaned up failed merge
 - added external movie programs to docs (closes #2490)
 - added all the external libraries that are include in the source
   tree
 - added six as explicit dependency.  This was discussed before and
   rejected, but I added it here because we now have a minimum version
   check which should be documented.
Merge pull request #2950 from tacaswell/install_dep_doc

DOC : edits to optional dependencies
Merge pull request #2922 from tacaswell/boxplot_manage_xlim

ENH : add flag to box_plot and bxp to manage (or not) xticks
PGF backend: Fix vertical positioning of multi-line text

Because text is drawn line-by-line, the anchor point of the text object
being drawn is not applicable to all lines.
When multi-line text is detected, apply the vertical alignment to the
anchor manually using the requested x and y positions.
This is based on the code in RendererSVG._draw_text_as_text.