Migrate to pytest and implement code coverage.

[sdhutchins]

Fixes issue #25

Changes proposed in this pull request:

• Migrate to pytest from nose given nose is no longer actively maintained.
• Added pytest.ini
• Implemented code coverage in .travis.yml
• Deleted unneeded mpi tests.
• Fixed syntax warnings.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev-master@93bb265). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             dev-master     #167   +/-   ##
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  Coverage              ?   15.13%           
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  Files                 ?       67           
  Lines                 ?     4182           
  Branches              ?        0           
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  Hits                  ?      633           
  Misses                ?     3549           
  Partials              ?        0           

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