Migrate express and AbstractRepresentation type from QuantumSymbolics

[apkille]

QuantumSavory/QuantumSymbolics.jl#92

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 17.04%. Comparing base (3d63b38) to head (fc420c3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/express.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   17.29%   17.04%   -0.26%     
==========================================
  Files          12       13       +1     
  Lines         399      399              
==========================================
- Hits           69       68       -1     
- Misses        330      331       +1     

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[apkille]

Just a note: I moved

express(obj) = express(obj, QuantumOpticsRepr()) # The default representation

to here from QuantumSymbolics to avoid type piracy in QuantumSymbolics. This isn't urgent whatsoever, but I wonder how beneficial it is to have a default representation for express, given that the ecosystem of packages supported by QuantumInterface is growing. This might be a bad comparison, but it's kind of like if DifferentiationInterface were to make its default backend AutoForwardDiff because ForwardDiff.jl has been around a long time and is used by a lot of people. Those things are true and ForwardDiff.jl is quite a handy package, but there's a ton of other autodiff packages that someone might rather use instead. I think the same could be said here. It probably would be a clearer translation interface if the user always has to explicitly define what backend they want to plug into. Of course, this would be a breaking change, but I thought I should put that thought out there.