Split ExplicitActionInterface to support optical data (#1160)
* Split ExplicitActionInterface in ExplicitCoreActionInterface and future alternative. ActionSequence::execute is now templated on the Params/State type and cast them to the concrete type.
* Add possibly missing include * Add missing template keyword * Move using statement down a level * Apply review suggestion
* Add and import `ImportWavelengthShifting` data * Update GeantImporter test harness * Import wls mean number of photons and time constant * Expand GeantImporter test * Update celer-dump-data; add imported data to the RootInterface linkdef