Improve documentation regarding setting up local epic and/or ip6 build

[rahmans1]

Environment: (where does this bug occur, have you tried other environments)

• Which branch (main for latest released): main
• Which revision (HEAD for the most recent): HEAD
• Any specific OS or system where the issue occurs?
• Any special versions of ROOT or Geant4?

Steps to reproduce:

The documentation in epic readme is not clear on setting up custom epic/ip6 geometries. If all commands are executed in sequence, after executing the final step source install/setup.sh , you will see the warning that ip6 is not loaded. This implies that local ip6 has not been picked up.

Desired behavior:

The warning should not show and you should be able to set custom epic or ip6 geometry using the setup scripts following the read me. The following setups seem to be allowed after cloning epic and ip6 under the same parent directory and could be added in documentation to improve clarity.

• Default epic+local ip6

source /opt/detector/setup.sh
git clone https://github.com/eic/ip6
cd ip6
cmake -B build -S . -DCMAKE_INSTALL_PREFIX=install
cmake --build build
cmake --install build
source install/setup.sh

• local epic+default ip6

source /opt/detector/setup.sh
git clone https://github.com/eic/epic
cd epic
cmake -B build -S . -DCMAKE_INSTALL_PREFIX=install
cmake --build build
cmake --install build
source install/setup.sh

• local epic+local ip6

git clone https://github.com/eic/ip6
cd ip6
cmake -B build -S . -DCMAKE_INSTALL_PREFIX=install
cmake --build build
cmake --install build
cd ..
git clone https://github.com/eic/epic
cd epic
cmake -B build -S . -DCMAKE_INSTALL_PREFIX=install
cmake --build build
cmake --install build
cd install/share/epic
ln -s ../../../../ip6/install ip6
cd ../../..
source install/setup.sh

N.B.: Maintain consistency between setup script in the container and setup scripts in local repos

Example : The setup script under /opt/detector uses $JUGGLER_BEAMLINE_CONFIG_VERSION whereas the setup script in ip6 local repo install uses $JUGGLER_BEAMLINE_VERSION. Need to make sure that the variable names are consistent.