✨ Convenience features

[burgholzer]

This PR adds some convenience functions that allow easier creation of QuantumComputations.
Specifically, a QuantumComputation can now be easily converted to an Operation. This allows one to create compound operations by first building a quantum circuit and, then, turning it into an operation.
Furthermore, a convenience function for adding classically-controlled operations to circuits is provided. This avoids the verbose code of having to create the quantum operation, constructing the ClassicControlled operation out of that, and then emplacing the resulting operation in the circuit.

Furthermore, it adds a method to determine the circuit depth.

Most of the changes here are actually in the tests, which have been refactored to take advantage of the new convenience features and, imho, look much nicer and less verbose now.

[codecov]

Codecov Report

Merging #226 (3b3ca8b) into main (64a8919) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #226   +/-   ##
=====================================
  Coverage   84.6%   84.6%           
=====================================
  Files         51      51           
  Lines       8715    8761   +46     
=====================================
+ Hits        7374    7419   +45     
- Misses      1341    1342    +1     

Impacted Files	Coverage Δ
include/operations/NonUnitaryOperation.hpp	92.0% <ø> (-4.5%)	⬇️
include/operations/Operation.hpp	93.6% <ø> (-0.3%)	⬇️
include/QuantumComputation.hpp	94.4% <100.0%> (+0.2%)	⬆️
include/operations/CompoundOperation.hpp	82.1% <100.0%> (+1.0%)	⬆️
src/QuantumComputation.cpp	70.1% <100.0%> (+0.2%)	⬆️
src/algorithms/QFT.cpp	100.0% <100.0%> (ø)
src/algorithms/QPE.cpp	86.7% <100.0%> (-0.2%)	⬇️
src/operations/NonUnitaryOperation.cpp	75.1% <100.0%> (+0.4%)	⬆️
src/operations/Operation.cpp	87.7% <100.0%> (+0.7%)	⬆️
test/unittests/test_qfr_functionality.cpp	100.0% <100.0%> (ø)
... and 4 more

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[hillmich]

LGTM! Avoiding the boilerplate for classically controlled operations is really nice.