Improve packaging / Minimum example

[schmoelder]

Hi there, your library looks quite promising, however, there seems to be some issues in the packaging / dependencies.

When running the example from the README, I get the following error:

  File ~/code/CADET-Process/untitled5.py:4
    from justpyplot import justpyplot as jplt

  File ~/software/miniforge3/envs/dev/lib/python3.12/site-packages/justpyplot/justpyplot.py:7
    from perf_timer import PerfTimer

ModuleNotFoundError: No module named 'perf_timer'