Feature/repr png more

[jinsoo960]

Description

_repr_png_() and plot() have been added for GaussianDensity when 2D and LocalQl, LocalQlNear, LocalWl, and LocalWlNear.
Also, an issue of getting additional empty plot when using PMFTXY2D in Jupyter Lab has been fixed.

Motivation and Context

Resolves #314.

How Has This Been Tested?

Tests have been added and the images are available as screenshots.

Screenshots (if appropriate):

Screenshots in #314.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the Changelog.

[bdice]

(Sorry, I started reviewing this before it was marked as "ready for review" - my mistake.) Let's install matplotlib on CircleCI so that it can try to render the plots, just toss it into this PR. It will make our tests run more of the real code paths that we expect.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d6184e9). Click here to learn what that means.
The diff coverage is 89.47%.

@@           Coverage Diff           @@
##             master   #318   +/-   ##
=======================================
  Coverage          ?    94%           
=======================================
  Files             ?     16           
  Lines             ?   3301           
  Branches          ?     19           
=======================================
  Hits              ?   3103           
  Misses            ?    186           
  Partials          ?     12

Impacted Files	Coverage Δ
freud/voronoi.pyx	95.75% <ø> (ø)
freud/pmft.pyx	98.72% <ø> (ø)
freud/environment.pyx	88.05% <ø> (ø)
freud/msd.pyx	75.7% <ø> (ø)
freud/cluster.pyx	94.16% <ø> (ø)
freud/common.pyx	95.89% <100%> (ø)
freud/density.pyx	97.06% <100%> (ø)
freud/plot.py	80.88% <79.48%> (ø)
freud/order.pyx	97.18% <93.1%> (ø)

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[bdice]

Suggested comments below.

[bdice]

Some suggested changes below. Thanks!