TrigT1CaloMappingTools: Make mapping methods const.

Declare IL1CaloMappingTool::mapping() as const.
Also update tools to use extends<>.

Further notes:

PpmCoolMappingTool relies on a BeginRun incident, and so is not thread-safe.
It should work as long as the run doesn't actually change during processing,
but fixing things for the general case will need further work.

PpmCoolOrBuiltinMappingTool should work, but it really should determine
its configuration during initialize() rather than during event processing.

Removed caching in PpmMappingTool.  Instead, restructure to use
arrays rather than std::map.