Define system environment

We should define a common Unix platform for as many systems as possible. This is the distribution and version, plus additional packages. I don't think we really care about the particular platform (Redhat-based, Debian-based, etc.) . We do have some special requirements, which we should declare up front. Some:

• gcc 6.3.0+ – mostly for DRP.
• python 3.6+ – could be from the system distribution as long as it conforms to INSTRM-306
• we need to check OS version requirements for the various hardware drivers. QuickUSB and rtdADIO are ok with 3.16; I need to check 4.x. MCS CameraLink chyan?

Please add to this ticket if you think of requirements. As few as possible but no fewer....