Fixing minor bug in disjoint_2q_block Layering and adding option add_barriers_between_layers

[EliasLF]

Description

This PR fixes a minor bug in disjoint_2q_block Layering, where it previously pushed 2q-gates erroneously to the second layer, if the first layer has 1q-gates already applied to the same qubits.

I also took this as an opportunity to separate disjoint_2q_block and disjoint_qubits into 2 different functions for the sake of readability. It was already getting cluttered now, and with a soon-to-come suggestion for another similar layering technique (specifically designed for noise-aware mapping) the problem will probably only get worse.
In my opinion, the ensuing code duplication is not too bad, but if you disagree, I am also happy to reintegrate both functions back into one.

Additionally, I added the mapping option add_barriers_between_layers intended for making layer boundaries visible after the mapping process.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b20ddfa) 93.0% compared to head (f358ec1) 93.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #405   +/-   ##
=====================================
  Coverage   93.0%   93.0%           
=====================================
  Files         46      46           
  Lines       4875    4895   +20     
  Branches     843     849    +6     
=====================================
+ Hits        4534    4554   +20     
  Misses       341     341           

Flag	Coverage Δ
cpp	92.7% <100.0%> (+<0.1%)	⬆️
python	96.0% <100.0%> (+<0.1%)	⬆️

Files	Coverage Δ
include/Mapper.hpp	83.8% <ø> (ø)
include/configuration/Configuration.hpp	100.0% <ø> (ø)
src/Mapper.cpp	90.3% <100.0%> (+0.4%)	⬆️
src/heuristic/HeuristicMapper.cpp	93.9% <100.0%> (+<0.1%)	⬆️
src/mqt/qmap/compile.py	97.1% <100.0%> (+<0.1%)	⬆️

[burgholzer]

Many thanks. Looking good ✅
Stumbled over two minor things that should be easily addressed or disregarded.