Bits for the cross-experiment (ATLAS, CMS, LHCb) study on heterogeneous workflows

This is needed for this study.

• extend the Avalanche Scheduler test suite with a CMS reconstruction scenario assembled by Christopher Jones (CMS);
• tweak the Gaudi execution flow simulator to be able to run the CMS scenario;
• update the LHCb Brunel scenario (new precedence rules and timing provided by @clemenci);
• add script which determines the critical path by a precedence trace;
• ... expecting more changes to come.

CC @leggett @clemenci