Move all atom files into a single-level directory layout

[odow]

Now that we can support arbitrary MOI sets, and we aim to move as much of the complexity to MOI as possible, including various bridge reformulations, it doesn't really make sense to talk about exp_cone atoms or second_order_cone atoms. (Consider the GeoMeanAtom, which can have a SOCP or PowerCone formulation.)

This PR moves everything to src/atoms/FooBarAtom.jl.

Longer term, I want to better document each atom, and provide a way of searching for various atoms that you can use.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.84%. Comparing base (cc6a9b0) to head (762e917).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #609   +/-   ##
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  Coverage   97.84%   97.84%           
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  Files          88       88           
  Lines        5066     5066           
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  Hits         4957     4957           
  Misses        109      109           

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