Support multiple containers in eic-shell

Right now we can only support a single container at a time in eic-shell. As we develop additional environments (focused on AI/ML, production, etc), we should have a way to run those seamlessly, e.g. `eic-shell --container ml --version 23.04.0`. This, in turn, means that the installation and validation functionality needs to go into eic-shell, not install.sh. In fact, it means that install.sh would ONLY install eic-shell, branch to docker vs singularity,, and then use eic-shell for the actual container pulling with argument passthrough.

Steps:
- move from $1 and shift to using getopt so we don't eat all the arguments
- eat those arguments that are not supported by eic-shell (prefix etc), but keep the others
- pass $@ to install_singularity and install_docker
- move the image installation into the bundled eic-shell script
- remove support for singularity version 2
- maintain the eic-shell for singularity and for docker as separate scripts in this repo (allowing CI)
- merge scripts in CI publication to the eic.github.io/eic-shell location