Fair2021 dev adaptation

DO NOT REMOVE THIS BOXED MESSAGE

Code in this merge request is subject to review by Code Owners.
The list of reviewers is posted in the comments section.
After all of the code is approved, please put a brief notice about it at the beginning of this message.

NOTE: Merging this code without getting it approved by Code Owners will result in suspending your developer rights!!!

THIS IS WORK IN PROGRESS
waiting for Fair suite Apr21 docker images: projected timeframe when ready ~ 22.10.2021
(CI fails, as docker images have june19 Fair suite installations)

This is Maxim's fair2021 branch with:

- updated to dev (there was a major update there with removal of unused detectors)
- significantly simplified installation script of FairSoft/FairRoot vApr21 scripts/install_fairApr21.sh
(this simple script is for common user)
- if we agree to merge this request, Readme.md should be updated after with information about Apr21 Fair suite install
(it is in the install script)