Paolo proposed cleanups after running Example 1

Hi everyone, I run Example_1 on the latest dev code and found a few "imperfections". Nothing serious, just a few printouts and cryptic error messages that can confuse the innocent user. For example acorn eval track_building_eval.yaml FRNN is not available, install it at https://github.com/murnanedaniel/FRNN. Using PyG radius instead. 100%|███████████████████████████████████████████████████████████████████████████████████| 10/10 [00:03<00:00, 3.29it/s
] /pscratch/sd/c/calaf/ITk/testarea/acorn/acorn/stages/track_building/utils.py:225: RuntimeWarning: divide by zero encoun
tered in log10 np.log10(varconf["x_lim"][0]), np.log10(varconf["x_lim"][1]), 10 /global/homes/c/calaf/acorn/lib/python3.9/site-packages/numpy/core/function_base.py:151: RuntimeWarning: invalid value
encountered in multiply y *= step /global/homes/c/calaf/acorn/lib/python3.9/site-packages/numpy/core/function_base.py:161: RuntimeWarning: invalid value
encountered in add y += start /pscratch/sd/c/calaf/ITk/testarea/acorn/acorn/utils/plotting_utils.py:39: RuntimeWarning: invalid value encountered in
long_scalars average = passed / total QStandardPaths: error creating runtime directory '/run/user/18228' (No such file or directory)