Provide multiple versions for end-point software

[wdconinc]

We should provide multiple versions of packages like g4e.

Rationale

There are user needs for multiple versions of end-point software (e.g. #98 (comment)), but likely no demands for specific underlying large dependencies (root, geant4). We should push multiple versions of selected endpoint software packages.

Description

Because we use environments to force installation of consistent minimal sets, we are typically restricted to single versions of a software package. We can probably use the matrix mechanism to increase the number of installed sets, while keeping the number of installed dependencies small.

General information

• I have run spack --version and reported the version of Spack
• I have searched the issues of this repo and believe this is not a duplicate