Make IdentityPlan! subtype AbstractCustomPlan

[glwagner]

Should close #45

[glwagner]

Perhaps we should add a test for NoTransform!?

[jipolanco]

Thanks for catching that! Yeah, I'll add a test for this.

[codecov]

Codecov Report

Merging #46 (9cc0a64) into master (4e0cdfe) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #46   +/-   ##
=======================================
  Coverage   98.32%   98.32%           
=======================================
  Files           9        9           
  Lines         419      419           
=======================================
  Hits          412      412           
  Misses          7        7           

Impacted Files	Coverage Δ
src/Transforms/no_transform.jl	100.00% <100.00%> (ø)

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