Refactor load_img

[emirkmo]

This is a full refactor of load_img to a more object oriented approach. All instruments are now classes that inherit from a generic Instrument parent/base class.

This now makes it much easier for me to add a few more fixes for image issues associated with edges. But before I do that, I wanted to merge a full working copy that reproduces the previous load_img functionality and makes it arguably easier to add new instruments.

The plan is to move most of this to an instruments.py file, and keep load_img.py a much smaller file that actually loads images. But that can be left to the future after the edge checking has also been implemented.

I will open some issues to track the other upcoming refactors.

Passing tests locally on python 3.7+ and a manual test of the run_photometry file. (Which we should also add as a unit test?)

Two caveats: we use spaces instead of tabs. This just made it much easier to read these multiline conditionals. I suggest we move to default 4 spaces following most major libraries and python black, as I would like to eventually enable it for automatic code formatting (to reduce code formatting maintenance overhead).

(I did a rebase onto devel before the merge to run the tests and run_photometry as flows table format had changed)

Fixes #29

[codecov]

Codecov Report

Merging #58 (b0b6839) into devel (dfe9d28) will not change coverage.
The diff coverage is 0.00%.

❗ Current head b0b6839 differs from pull request most recent head 2ff032e. Consider uploading reports for the commit 2ff032e to get more accurate results

@@          Coverage Diff          @@
##           devel     #58   +/-   ##
=====================================
  Coverage   0.92%   0.92%           
=====================================
  Files         26      26           
  Lines       2281    2281           
=====================================
  Hits          21      21           
  Misses      2260    2260           

Impacted Files	Coverage Δ
flows/load_image.py	0.00% <0.00%> (ø)

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