Add performance tests

[akrzgc]

This branch is adding performance tests for nanoDFT.

So far it is possible to just execute initial 5 tests with simple purest command purest tests/test_benchmark_performance.py.

The code injecting the ipu_cycle_count into the _nanoDFT() jitted method is WIP.

The branch is going to:

• [x] add nanoDFT() performance evaluation with ipu_cycle_count
• [x] use pytest for performance testing
• [ ] add new fullerene molecules such as C80 or C100
• [ ] consider using GitHub Action benchmark for performance tracking (either already within this branch or as a separate change)
• [x] refactor the ipu_cycle_count code and move it to utils