Use pandas.concat() instead of frame.append() where needed

When running the visualization step I got:

/home/gk/venv/lib/python3.9/site-packages/OnionPerf-0.9-py3.9.egg/onionperf/visualization.py:55: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  main_df = main_df[ind].append(df[ind2])

We should check the whole onionperf code base for that issue and fix it (and maybe requiring a min pandas version, not sure; it seems the deprecation started with pandas 1.4.0).