More Image generalizations

[kbarbary]

More changes to Image to be agnostic with respect to SDSS.

• remove run, camcol, field info from Image (turns out these were not really used anywhere)
• remove dimensions info from Image constructor (redundant with size() on the pixels argument).

• make Image parametric in the "sky" attribute. Formerly the sky had to be SkyIntensity which was specific to the SDSS data model. Now, it can be any AbstractMatrix.

  SDSSBackground (formerly SkyIntensity) is now a AbstractMatrix:

  
  julia> using Celeste

julia> bkg = Celeste.SDSSIO.SDSSBackground(fill(1., 1, 1), ones(5), ones(5), fill(3., 5)) 5×5 Celeste.SDSSIO.SDSSBackground: 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

[codecov[bot]]

Codecov Report

Merging #717 into master will decrease coverage by 0.02%. The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #717      +/-   ##
==========================================
- Coverage   81.26%   81.24%   -0.03%     
==========================================
  Files          35       35              
  Lines        3641     3642       +1     
==========================================
  Hits         2959     2959              
- Misses        682      683       +1

Impacted Files	Coverage Δ
src/AccuracyBenchmark.jl	80.85% <100%> (-0.06%)	:arrow_down:
src/model/image_model.jl	25% <100%> (-50%)	:arrow_down:
src/SDSSIO.jl	93.95% <95%> (+0.03%)	:arrow_up:

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[jeff-regier]

All nice changes!