Specification

This MR is a major undertaking, that we've been working on for a long time, and as such all the changes are difficult to capture in a description.

All changes have been separately reviewed in smaller MRs, so the entire code change is considered already reviewed.

For future readers' sake, this MR deprecates the e3.bash distribution method and replaces it with specification handlers, that only can install entirely defined EPICS environments. See the repo https://gitlab.esss.lu.se/e3/specifications for examples of these definitions