Add the possibility of loading the conditions overlay from existing YAML files
The conditions overlay was introduced with !72 (merged) to support LHCb Alignment application.
It turned out that there were a number of missing features needed to support the way the alignment procedure works in the online system, in particular the possibility of initializing the overlay from the content of a directory, for example generated by an overlay dump). The idea is that alignment jobs will communicate between each other the up-to-date alignment constants via a shared directory that will be both the target of the dump and the base for the initialization.
This MR add that functionality together with a detailed test of the foreseen workflow.
I also added a number of minor fixes and improvements needed by and LHCb application that wants to use the new feature, in particular I changed the way DetectorDataService::initialize
gets the initialization parameters.
This should go with LHCb!3705 (merged)