Combined PAH feature strengths

[alexmaragko]

This PR calculates the combined PAH feature strengths, and should replace #142.

Differences with #142:

• Combined PAH feature strength function is now inside feature_strengths.py module and called in base.py.

An EQW column is created in the output ipac table as placeholder for the next PR (that will replace #143).

[codecov-commenter]

Codecov Report

Merging #162 (9f58f47) into master (04d7b08) will decrease coverage by 0.86%. The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   55.79%   54.92%   -0.87%     
==========================================
  Files           8        8              
  Lines         466      477      +11     
==========================================
+ Hits          260      262       +2     
- Misses        206      215       +9     

Impacted Files	Coverage Δ
pahfit/helpers.py	43.33% <ø> (ø)
pahfit/feature_strengths.py	31.57% <20.00%> (-12.87%)	:arrow_down:
pahfit/base.py	56.78% <33.33%> (-0.29%)	:arrow_down:

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[alexmaragko]

Last commit includes minor docstring updates, to address issues #157 and #137, and an update to the output table in fit_spectrum.rst to address #160.

Most of the test fail now because sphinx is not able to generate the rst file.

I have copied the ipac output table to the rst file, but it seems formatting issues appear. I'm not sure what is the quickest way to deal with this in order to avoid formating line by line.

I have also tried the astropy ascii.rst format output which generates the table's body properly when pasted in the rst file but not the header.

[alexmaragko]

Fixed the formatting issue by using the ascii.rst format output. Now the matrix test seems to be failing due to Python 3.6 setup with tox environment on macos.

[karllark]

Will work on a separate PR to update the text dependencies.

[karllark]

Test failures fixed in PR #163. Best solution would be to rebase this PR once that PR is merged. That would allow the full tests to be run. Ask and I can help through the 4-5 commands needed to do a rebase (should be clean and straightforward for this PR).

[alexmaragko]

Rebased the PR and changed PAH_112_C to PAH_113_C.