Fill mutable in L1ParticleMap in a thread-safe way

L1ParticleMap delays filling indexCombos_ until requested. This
could have been changed to being done in the constructor. However,
any data already stored may or may not have had indexCombos_ updated
(since indexCombos_ is stored) and therefore we	still must do a
delayed	fill. This is made thread safe via the use of a std::atomic
which keeps track of the state of indexCombos_. The new std::atomic
is declared transient and placed next to bool member data in order
to avoid extra padding by the compiler.