Allow rootlogon to run without /home/trholmes, and adjust README to match scripts docstrings

[alexandertuna]

cc @trholmes 🍉

When running makeMuonPlots_k4hep.py:

1. I bumped on /home/trholmes in .rootlogon.C, which I didn't have access to.

Apptainer> python makeMuonPlots_k4hep.py
Running rootlogon.C
Error in <TROOT::LoadMacro>: macro /home/trholmes/utils/AtlasStyle.C not found in path .:/opt/spack/opt/spack/linux-ubuntu22.04-x86_64/gcc-11.3.0/root-6.28.02-67zd35p6a6ujklkrec5whddkmbsekypu/share/root/macros
input_line_37:2:3: error: use of undeclared identifier 'SetAtlasStyle'
 (SetAtlasStyle())

2. The apptainer build/run commands in the README right now are a mix of relative [0] and absolute [1] paths for the .sif file. I bumped on this because I ran apptainer build in some random directory. The scripts [2] which mention apptainer use relative paths, so I changed README to use relative paths, for consistency.

[0] apptainer build uses k4toroid.sif
[1] apptainer run uses /home/$USER/mucol/k4toroid.sif
[2] pfoStudies.py, makeMuonPlots_k4hep.py, electronStudies.py

PS. Please forgive my insufferable use of "I bumped on"