Add classmethod to instantiate ChemKED from ReSpecTh

[bryanwweber]

• [x] Fixes #62.
• [x] Tests added
• [x] Added entry into CHANGELOG.md

Changes proposed in this pull request:

• PEP-8 some of the test code (except for line length). This makes it easier to spot tests that don't test anything
• Fix the coverage gap opened up in #95
• Add a method to instantiate a ChemKED class directly from a ReSpecTh XML file. The CLI scripts still write the file output
  • Improve the tests for the CLI scripts

@pr-omethe-us/chemked

[codecov[bot]]

Codecov Report

Merging #96 into master will increase coverage by 0.22%. The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #96      +/-   ##
========================================
+ Coverage   99.77%   100%   +0.22%     
========================================
  Files           5      5              
  Lines         875    883       +8     
  Branches      209    210       +1     
========================================
+ Hits          873    883      +10     
+ Partials        2      0       -2

Impacted Files	Coverage Δ
pyked/chemked.py	100% <100%> (ø)	:arrow_up:
pyked/converters.py	100% <100%> (+0.62%)	:arrow_up:

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[bryanwweber]

@kyleniemeyer This is GTG on my end. If it looks good to you, feel free to merge