Allow `source setup.sh epic_arches` to avoid need for DETECTOR_CONFIG overrides

**Is your feature request related to a problem? Please describe.**
Explaining to users how to load the geometry currently requires that the detector geometry is loaded with `source setup.sh`, and then the DETECTOR_CONFIG variable needs to be overridden from its default value of DETECTOR. This is arcane and error prone (non-existent configs can be specified). I would like to support the syntax `source setup.sh epic_arches` or `source setup.sh arches` to set DETECTOR_CONFIG immediately to `epic_arches`. This would also check that the config actually exists and print a warning if it doesn't.

**Describe the solution you'd like**
Something like `${1:-@PROJECT_NAME@}` in the setting of DETECTOR_CONFIG, and a check for `test -f $DETECTOR_PATH/$DETECTOR_CONFIG.xml` with warning.

**Describe alternatives you've considered**
We can keep the status quo.