Closes #3 - adding Toffoli (CCNOT gate)

[MKorp7]

Overview

Adds native Toffoli (CCNOT) gate to QANDLE. Tested against pennylane manually, added automated testing of circuit splitting

Closes #3

Motivation

As mentioned in issue: "The Toffoli gate is essential for universal quantum computation and widely used in quantum algorithms and error correction."

Changes

• Core: qandle/operators.py – CCNOT implementation
• Drawer: qandle/drawer.py – drawing of circuits with CCNOT
• Splitting: Toffoli decomposition based on
  Shende, V. V., & Markov, I. L. (2009). On the CNOT-cost of Toffoli gates. Quantum Inf. Comput., 9(5), 461-486
• Tests: tests/test_ccnot_split.py – 3- and 10-qubit circuits with CCNOT splitting

Checklist

• Checked performance of CCNOT againt pennylane implementation
• Splitting implemented and tested
• New test for pytest, passes
• Code style mirrors existing CNOT implementation
• Docs updated