Standardize units

[jdtsmith]

What

This introduces a new file units.py which standardizes the units for science-packs, and eventually for the entire Features->Model->Fitter chain.

Why

This will:

1. make a proper AREA-based Gaussian/Drude fitter possible (see #225)
2. standardize the report of value in results Features tables
3. ensure most values are within say 5 dex of unity for table glance-ability and to helping with underflow.

Note: we will convert any user input units into this format, and optionally convert tabulated/plotted output to the user's spectral units. But for specifying the PAHFIT science model to be fitted itself, these are the working units [1].

See this comment for information on how we can leverage astropy.units to understand user-input spectral units, e.g. to discriminate between flux density and spectral intensity (aka surface brightness).

Proposed PAHFIT working + output units summary:

Type	Working Unit
Wavelength/FWHM/spectral_axis[2]	µm
Temperature[2]	K
Flux density	mJy
Flux power	1e-22 W/m2
— OR —
Surface Brightness/Intensity	MJy/sr
Surface Brightness Power	1e-10 W/m2/sr

[1] Internally, fitters may adopt any different units for convenience or other reasons, but that is a hidden implementation detail. For the user, these will be the working units. [2] These are already effectively standards in our science packs

[codecov-commenter]

Codecov Report

Merging #259 (296abe2) into master (89baf2d) will increase coverage by 0.40%. The diff coverage is 93.54%.

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@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   64.78%   65.19%   +0.40%     
==========================================
  Files          14       15       +1     
  Lines        1045     1063      +18     
==========================================
+ Hits          677      693      +16     
- Misses        368      370       +2     

Impacted Files	Coverage Δ
pahfit/features/features.py	74.50% <90.90%> (+0.15%)	:arrow_up:
pahfit/units.py	100.00% <100.00%> (ø)

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[jdtsmith]

Note that this supersedes #241 ; the tied-features component of which will be submitted as a separate PR after this gets merged.

[jdtsmith]

An extended version of this PR is now in #284.