2D Background Estimation

[mileslucas]

This PR is working on adding the 2D meshing for the background and a zoom interpolater.

This has changed some of the interface for the BackgroundEstimators and has added BackgroundRMSEstimator with implementations for Std MadStd and BiweightScale.

[mileslucas]

Todo:

• mesh creation
• crop mode
• pad mode (array creation is fine, but NaN math breaks it)
• zoom interpolator
• tests for the above
• docs for the above
• figure out a way to do iterative sigma-clipping within estimate_background call.

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 2.45%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   95.17%   97.62%   +2.45%     
==========================================
  Files           9       10       +1     
  Lines         580      631      +51     
==========================================
+ Hits          552      616      +64     
+ Misses         28       15      -13     

Impacted Files	Coverage Δ
src/Aperture/Aperture.jl	100.00% <ø> (ø)
src/Aperture/circular.jl	96.29% <ø> (ø)
src/Aperture/elliptical.jl	98.73% <ø> (ø)
src/Aperture/overlap.jl	95.92% <ø> (ø)
src/Aperture/plotting.jl	100.00% <ø> (ø)
src/Aperture/rectangle.jl	95.74% <ø> (ø)
src/Background/Background.jl	100.00% <100.00%> (+25.00%)	⬆️
src/Background/estimators.jl	100.00% <100.00%> (+32.35%)	⬆️
src/Background/interpolators.jl	100.00% <100.00%> (ø)
... and 1 more

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