WIP: Configuring hypo for multi-part jet chains

FYI @jbossios, @peter, @fpastore, @tbold

I tried to see what minimal changes would get the multi-part jet chains correctly configured, and this attempt worked, without changing the rest of the menu. Not sure it is 100% ideal, but maybe we can look at it as a starting point (bearing in mind of course the changes in !27770 (merged) that need to be finalised and will certainly conflict).

There are two main changes:

1. A ChainConfiguration item may hold a list of chainParts rather than just one, but would usually just be initialised with a single one.
2. The decodeHypoToolConfs function gets an exception for jet chains that just hands over the full chain dictionary

See these files (the other changes are client updates and a few inserted comments to document jet chains and chainParts): Trigger/TriggerCommon/TriggerMenuMT/python/HLTMenuConfig/Menu/ChainConfigurationBase.py Trigger/TriggerCommon/TriggerMenuMT/python/HLTMenuConfig/Menu/ChainDictTools.py Trigger/TriggerCommon/TriggerMenuMT/python/HLTMenuConfig/Menu/MenuComponents.py