Version 0.6.0: performance improvement, modern C++20 and matrices

Almost a complete rewrite. No functional change except:

• error models don't work anymore :( they will come better and stronger later on
• classical not gate doesn't work anymore :( it's just an omission, will be fixed soon
• get_measurement_outcome and get_state don't exist anymore :( they've been superseeded by json output
• it's faster and using a single core \o/ and less memory on average

Added

• Ability to easily implement any quantum gate by simply giving an arbitrary unitary matrix in Gates.h
• Performance speedup using a single thread
• Some proper linear algebra primitives using template metaprogramming
• Abseil-cpp dependency for flat_hash_map
• Ability to compile the simulator with an set maximum number of qubits - in the future this will
  be using templates as well to switch data structures based on the input quantum circuit

Changed

• Quantum state is stored as a sparse array implemented using a flat_hash_map from Abseil-cpp
• C++20 in place of C++14,
• Code style: camel case

Removed

• OpenMP and multithreading.
• SSE, AVX and other intrinsics.
• get_measurement_outcome and get_state methods. Now accessible via JSON output.